That 9800 GT you mentioned in your other thread would work, if it's in your budget and if your computer's power supply will support it. I actually just ordered one for USD$99 on newegg.com
this past weekend and should receive delivery in the next couple of days. It comes with a $30 mail-in rebate if you want to take advantage of that as well. This particular one from MSI doesn't require an external power connector so if you have at least 400 watt power supply in your computer you should be OK. I'm actually going to try mine on a computer with a 375 watt power supply to see if that works.
You could also use a GT 240
. They should work as well and don't require external power connection, though I've not personally tried it. Looks like they're running in the USD$70-$80 range and some have mail-in rebate too, which will drop the price down another $20-$30.