Advice on upgrading my system

Welcome to the forum fish

Well aside from the obvious which is that you may want to save for a new system soon i recon that your best bet for a straight GPU swap would be the 7600GT due to the low power draw on it.
A three year old 350watt psu isnt going to power much more than thatlowing for degredation.
Or if you want you could swap the psu to something with more clout and put a 1950 pro in it which would make a very sensable last upgrade for your machine.
Also consider if your mobo is compatable putting a cheap core 2 duo in.
And if you get a psu that is slightly over kill for this pc say 550/600watts then that and the c2d could be the start of your next machine
Mactronix

something like this http://www.overclockers.co.uk/show [...] subcat=928 posted by another forum member would be fine if you went for the 1950 option and would be quite capable of running more power hungry cards if you decided to gowith the keep it and reuse it in another build option.
Or if you just wanted something a bit cheaper to run the 1950 pro then this http://www.overclockers.co.uk/show [...] subcat=823 is a very good psu and to be honest is probably the best route as the cards are using less power hungry these days and this would easily handle a 8800GT or 3870.
Mactronix