Upgrading my desktop to be able to play Battlefield 3 on PC?

-I have a HP p6680t

Intel Core i5 CPU 750 @2.67GHz (4CPU's) ~2.7GHz's

Memory: 8192MB RAM

DirectX 11

Display :ATI Radeon HD 5450

Chip type: ATI display adapter (0x68F9)

DAC Type: Internal DAC (400Mhz)

Total Memory 222 MB

Current display mode: 1600 x 900 (32 bit) (60Hz)

Hardrive: 1 TB



I know I need to buy a new power supply and graphics card. I am willing to spend around $250 on the graphics card and $50-60 on the power supply. What type of power supply is compatible with my desktop? What's a pretty decent graphics card that will most likely run BF3 and at good setting and that is compatible with my desktop? I am new to PC gaming and I have no clue on how to pick a graphics card and what is needed to install it. Also, how do I find out if my graphics card is single slot or dual slot??First of all, how many watts is your current power supply unit? That graphics cards will probably run Battlefield 3 fine, I think you may not need to upgrade at all...