Personally I would keep 2 Gb in a 32 bit OS, you'll see little improvement for adding the extra 1 GB, and the extra 1 Gb can play havoc on some motherboards as 2x 512 stick with your current sticks, you general have to match the speeds of the current sticks as well so it can be a real headache unless you buy 4gb as 2 x2 Gb stocks and throw away your old ram!
THERE IS NOTHING wrong with adding 2x 2Gb sticks in XP 32, Sp3 will "see" all 4 Gb but only use upto 3 GB of the 4, there are a few games that can benefit a bit (but not many as XP is not the RAM hog vista is). Your GPU VRAM is included in the 4GB limit so 4 GB + 256-1024 VRAM normally equals 4GB+ 
you can always grab 4Gb now so your ready for Vista 64, but you see alot more benefit with 4Gb in V 64
A video card upgrade could net more FPS or smoother gameplay than adding extra ram to a 32 bit OS though. So a new GPu could net you a 100% gain in FPS and new ram 1-5%!
If you moved to vista 64 you bascially need 4 GB as vista eats 1GB+ just to run so you need the extra 3 Gb to fit most 2-3Gb games, quite a few games will eat over 2 Gb in Vista 64, crysis and Assains creed are examples, Sin eats 64% of my ram when playing (I have 4 GB).
