What's the PSU make & model number? Or, the PSU's total wattage & +12V rating(s) in Amps? E.g. this pic click here shows a Enermax 1000W PSU with 5x +12V rails each with 17A. And does it have 2x 6pin PCI-E power connectors?
How much space do you have in your case for a long(ish) graphics card? I'm looking for this measurement click here. Is it an ATX / mATX case, & do you have any other cards installed in the PCI/PCI-E slots?
If the above proves OK, then an ATI HD5850 click here would be the card to go for, even if it's a shade over your budget. But if you'd like to stick to your budget, then an ATI HD4890 / Nvidia GTX275 would be suitable. All 3 cards compared at TomsHardware click here. The HD4890 would be preferable as it's much cheaper than the GTX275 for similar performance. G