This is a quote "if a game is running at 1024 x 768 x 32 bit colour, it takes 3MB for the screen rendering (times 2 for double buffer) plus another 3MB for z-buffer which totals 9MB just for the screen. If you're target system has a 32MB video card then you're left with 23MB for textures. A single 1024 x 1024 x 32bit colour texture takes 4MB of video ram + 1.333MB for mipmaps, which gives you room for only 4 textures of that size (also remember that the HUD/GUI images take up texture memory as well). If you use textures of 512 x 512 then they are 1MB + 0.333MB each, which allows you 17 textures of that size."

So how do they do it for "Far Cry" and "Oblivion" with tons of plant models, weapons, characters, etc?

So how large do you make your textures to get some decent detail for a rpg? What if the player decides to drop many items in a scene like Morrowind or Oblivion?

first of all oblivion would never ever ever run on a 32 mg or even an 64 mg card, and will lag on an 128 mg card, so you have to look at the specs, second of all, most plants need a 256x256 pixel image at max, and you re use textures as well;

There is a such a thing as texture compression(DXT), which is similar to JPEG, in that the texture takes up less memory with a slight sacrifice in image quality. This is done on hardware and is ubiquitous.

And a 32 MB card is like...an antique. My card has 640 MB for instance.

mine has 256 and can take from the ram too as video memory

I had released a game 7 years back and kept a minimum requirement of a 32 mb card.
now mine has 128mb+512mb ram now and am going to upgrade to a beast soon...but still oblivion runs decent at lowest settings