Actually, the new generation cards from both makers (nVidia 6xxx and ATi x800, x600) blow away all older cards in every aspect. I wouldn't buy a nVidia 5xxx line, because they are not as much up to par against the ATi equivalent (9500+). Since the new gen cards are hard to find, and have very high price, I would definitely go for a ATi card 9500 or up if you are looking for something that is cheaper but still performs well. I have a Geforce ti 4600 that still runs well... that is when I actually played EQ heh.
Tom's Hardware Guide was mentioned earlier in the thread, and is a good resource for knowledge.
Pricewatch was also mentioned. It's more for ballparking the prices, but I have used both
newegg and
Zipzoomfly and have been satisfied.