[1500 char limit sucks]
Lawso, 1gb ram can be up'ed to 2. I manage at 1gb ram with a 6600GT.
I've been using this same system for about 5 years. (I upgraded from 512mb ram to 1gb start of this year - got some stuff second hand).
Skitch93, I do believe you build computers. Sorry, I'd never let you make me a computer.
On to the biz:
OSs... I use XP. Managed with 512mb for quite a long time, I'd suggest it as a minimum. Vista, I'd suggest 2gb ram. I'd suggest XP unless you have a system like what I've posted above... I'm happy with XP.
If your buying a new system, minimum you can really buy is a 2x1gb kit (2gb).
Your mobo can be checked in windows... go and download a program called CPU-Z. It tells you everything you need to know, and all the stuff you don't. Very easy to use.
6600GTs are the best you'll get on an AGP system, hence why I haven't upgraded (I need a whole new system). If I were to get a new system, I'd get something like the above +- a few a things maybe.