Q. What is "stuttering"? 

This term generally refers to sudden pauses or jerky behavior that temporarily drop your FPS rate into the dumpster. It's a common problem in graphics-intensive games like Oblivion, especially if you don't have enough VRAM to contain all of the textures and meshes that need to be rendered in a certain area of the game.

Q. What's the difference between normal maps and color maps? 

The color maps provide most of the color patterns you see in-game, while the normal maps add depth and shadows. Here are some examples: | Color map | Normal map |

Q. What does "LOD" mean?

LOD stands for "Load Objects Distant". It's an over-used term, but it generally refers to anything that can be seen in the distance (i.e., beyond your close/mid-range uGridsToLoad setting). 

Most of the time, when we talk about LOD here, we are talking about a specific folder called LandscapeLOD, which contains texture files (color maps and normal maps). In the Oblivion Data structure, the path is:

\Oblivion\Data\Textures\LandscapeLOD\

Q. What impacts FPS more: normals or textures of the same size?

Neither of them will impact FPS at all unless you exceed the VRAM limitations of your video card. In this respect there is no difference between color maps and normal maps -- they both have the same file size at the same resolution. Larger texture sizes take up more space (a LOT more space!). This means that the larger texture sizes may cause increased lag and stuttering if you don't have enough VRAM to contain them.

Q. How accurate is the approximate VRAM usage table on the Distant Landscape 1.2 page? Can it be true?

The numbers are very rough, for now. It's based mostly on the file size, but it's probably not far from the exact figure.

Here's my understanding of the way it works. Perhaps someone else with more knowledge can elaborate.

All of the LandscapeLOD textures (and any other distant object textures) get loaded into VRAM anytime you're outside, regardless of whether you can actually see them from where you are standing. The distant object textures that are actually being rendered at any given moment will be less than this, so if you don't have enough VRAM to hold them all then the inactive ones will be temporarily swapped out to main system memory or flushed entirely (depending on your system limits). This by itself can cause longer load transitions and increases stuttering as you move around the world, which is why Bethesda made the default distant textures so incredibly small and soupy.

The problem gets even worse because close-range textures loaded will vary at any point based on your uGridsToLoad setting and many other settings like object-fade, etc. These close-range textures have to constantly being loaded and unloaded as you move, further increasing the lag and stuttering when you move around, especially if you are running or riding a horse. In the worst case scenario, you'll get a complete slideshow because textures must be loaded and unloaded to render every single frame (i.e., not enough VRAM to handle just the visible elements being rendered). If you get into combat or walk through a city, then the problem becomes worse again because each actor may add new textures that must be rendered in each frame.

So, the answer to your question is not simple. It depends on a lot of factors. If you use vanilla textures for everything, at large texture setting you will frequently exceed 128Mb, which is why Bethesda recommends using a 256Mb card and people with smaller cards usually have to run at medium texture setting (uses the first mip-map, which is 1/4 the size of the main texture). If you have a 512Mb card, then you have plenty of room to spare over the vanilla textures, making it safe to add larger distant landscape textures, larger close-range textures, etc., without any performance loss. But, the vanilla distant landscape textures only take up about 4Mb. It doesn't take much of an increase in the dimensions of these textures to quickly eat up all your free space and then some. 

The million-dollar question is "how much more can you handle?" Mods like QTP3 dramatically increase the size of almost all the most common textures. In most cases they are at least four times as large as vanilla textures, and in some cases much larger than that (some of the uncompressed normal maps in QTP3 are more than 10x the size of vanilla textures). And, mods like OOO, MMM, and many, many others can all add even more to the texture burden on your system. So, you might be able to run QTP3 by itself with no impact on a 512Mb card, but most people have a lot of other mods running, too. If you add 276Mb worth of distant landscape textures and use the AEVWD mod on top of this, etc. ... well, then you've created a perfect recipe for a slideshow rather than a playable game.

I'd be very interested in a more detailed study of VRAM consumed by distant landscape textures, but it needs to be tested on a clean vanilla install with no other mods that add or replace any textures. If anyone is up to the task of producing a report like this, I will gladly incorporate the results into the TOTO pages (with full credit, of course).

One other anecdotal example may be useful here. Way back when I created the first reduced texture mod for QTP2, I reduced 300Mb worth of close-range normal maps to about 75Mb. This was a total savings in file-size of roughly 225Mb, yet only a fraction of this reduction would have an impact at any one time because they were all close-range textures (very few of them are active at the same time). The total savings in VRAM usage to a player standing still in the wilderness was probably only 25Mb (and maybe much less). Yet the overall performance impact (reduction of stuttering and lag) in average gameplay situations was dramatic.

Now compare that with a reduction by more than 200Mb in distant landscape textures -- which are always loaded anytime you're in any exterior cell -- and I think you're looking at a much greater impact on overall performance.

Ultimately, you'll just have to try it and see for yourself. Try with the 2048 textures (or even 1024 textures) for at least a while to see what sort of difference it makes. You can always switch back to the larger ones if you don't like the results.

Q. Where can I find more answers?

Check the Texture Help page or the ESF Forums (link on the TOTO main page).