Yeah, although watts don't mean everything, I can pretty much tell that even a high-quality 300W PSU can't be enough. Try to go for something with enough amps in the 12V+ line (or lines - if there's several lines, it's almost as simple as adding the lines together). Anything over 30A should be sufficient unless you're going for high-end cards, though more is better and makes future upgrades more likely to be possible. Let's make that at least 34A-36A at least, instead of 30A, just to be sure. I, for one, have 15A+15A (30A) for my GeForce 8800 GTS 320MB and my PSU was probably only barely enough.
Anyway, it'll cost more with a new PSU as well... But I guess that's not a surprise. And try to look for a decent manufacturer.
As for the new card, right now I guess GeForce 9800 GT/Radeon 4830/Radeon 4850 might be pretty good low-price cards. Not completely sure, though, as I've been "out" since DX10 came out.







