Just avoid any of the GeForce 5xxx series. They were some of the worst bang-for-the-buck cards. Also be aware that 6200 LE, and other LE cards steal system memory to do their graphics. If you can get a card that uses DDR3 memory all the better.
You might check out Tom's Hardware's
Best Video Cards for the Money, recently updated for December 2008. Note that it now exclusively covers PCI Express cards. If you need AGP, you'll have to go back about a year on his charts when he still covered AGP. Of the AGPs, the GeForce 6800 XT is by far one of the best. I have one, and still love it. Although it doesn't get to flex its EQ2 muscle like it used to, as the CPU is really too slow for it.