If the motherboard supports PC3200, I'd definitely go for that. Maybe a faster hard drive, as well? (7200.12 Seagates work well for me) As for video cards, I've bought about 15 of the ATI (Sapphire) 4650's over the past year or so, with no failures. Definitely ATI has seemed stronger recently, over Nvidia, but you know how it goes - ask a hundred different people, you'll get a hundred different answers on all these subjects. ;-)
Also price out a system from the Dell factory outlet. Certified refurbished Optiplex systems with the same warranty as new, for 1/2 the price. Honestly, if price of upgrading a used computer exceeds $300-$400, I just recommend purchasing a new one.
