If you can scrounge up the dough, I'd grab this
If not, this'll do you just fine!
EDIT: I hadn't even thought about PSU, like wolv. said. I check on newegg and that's why I didn't see it cuz they don't have the PSU listed. You've only got a 300W PSU according to HP, so you will need to bump up. I'd say go with the 9800GTX+ I recommended and this PSU
to stay close to your budget.
Intel Core i7-2600K @ 3.4GHz || GIGABYTE GA-Z68X-UD3P-B3 || Corsair Vengeance 16GB DDR3-1600 || EVGA GeForce GTX 480 || Samsung EVO 256GB SSD
my feet get cold and that gives me diarrhea sometimes - Smart_Guy