Thing to check before you splash out on a card is it’s maximum power draw, can your power supply unit supply enough Wattage for it in combination with all your other existing PC components? Also a lot of the medium to high end cards have to have additional PCI Express 6 pin and/or 8 pin power cables from your PSU to power them, as the card needs more power than the PCI slot can deliver on it’s own.
If you are going to be using it to watch video’s etc than it might be worth checking the reviews of your shortlist of cards as some cards can be really really noisy.
Then to complicate things a bit more some cards can be really long so it’s worth checking how long a card you can fit in your pc case.
I’d second Tinribz with Tomscharts being a good starting point. Nvidia 9600, 9800GT (aka 8800GT) or ATI 4830 are all in your pricerange.