Well for games I'd say ur already shooting urself in the foot with the GeForce FX. I have a 5900 Ultra which I got a year ago and wouldnt want to play any of the latest games on anything slower...But then I like my graphics :D. Doom 3 should run fine with slightly lower settings though.
Def go for 1GB RAM though it makes the world of difference! Faster RAM is obviously better and you may find it to be cheaper than getting new RAM of speed 266.
As far as storage goes it is all very cheap now. Rather than one drive I'd prob recommend two. Then you can have a back of ur stuff on the other one just incase, and put ur page file on a seperate drive to ur OS too. As well as the scratch pad for Photoshop.
Althon 64s are good. Check out a few review sites:
They all have some good reviews and links to some other good sites too.
I am a student and know where ur coming from. But you have a good budget there if you spend it wisely!