i'd definitely recommend getting another 1gb of RAM. make sure you plug it into the correct slot for the DDR to work (if you have 4 slots for RAM, as far as i know slots 1/3 and 2/4 are synched).
for your gfx card i'd go with the 7950GT. more memory does not necessarily mean a better card, as sometimes manufacturers will put 512mb on a card and clock it lower than a card with 256mb. the important things to look for on a gfx card, in addition to memory size, are the core clock, memory clock, and the memory interface. higher is obviously better on all of those.
as for your PSU, your machine doesn't appear to be a powerhouse so a good 400-450w PSU should be more than enough. make sure it has at least 18A (preferably 20A) on the 12V rail. you do not need dual 12V rails unless you are planning on using SLI/Crossfire. enermax and thermaltake are both good brands.this
one would do you well, i think
i hope this helps.
Edited by dogslikeus, 25 January 2007 - 12:10 AM.