Best graphics card for a HP pavillion p6345a?

It came with a ATI radeon HD 5450 and... It's junk. Can only run NFSMW on the LOWEST resolution and graphics. I've been looking at the AMD HD 7950 and it really interests me... however I'm quite sure my computer doesn't have the necessary requirements to even run that card.
Stats
-1 PCI express x16 slot
-Power supply is most likely in the 300's due to it being a HP and majority of HP's are in the 300's.
-I5 3.3hz processor
-4gb ram

The main thing I'm worried about the the PCI slot. I'm quite sure 1 slot is not enough and I have no idea how much they cost, or if you can even add more slots. Power supply isn't too expensive as far as I know. The thing I'm looking for is someone to tell me the requirements I must meet to run the AMD HD 7950, power supply, PCI slots, etc. I'm quite sure the needed power supply is about 550-600 watts(EDIT, I just googled harder and the requires supply is only 200-250 watts, woo!) So would anyone know if my computer is big enough to fit it?