Computer upgrade Help plzzz...(Giveaway pending)

Hey friends

The season of deals is coming up and I am looking to upgrade my PC. I got this configuration in 2010 (December 2010 to be specific) and till now it has worked perfectly.. but these days the games are more demanding and most of the games I play have to be played on lower quality for me. If I try to play the games on high quality, they really stutter. So I am guessing I would need a new Graphics card most probably. But I am not very knowledgeable in the hardware configuration area. SO looking for some help.

Here is the list of all the parts that I had bought at that time.

SAMSUNG CD/DVD Burner Black SATA Model TS-H663C/UOAH - OEM

LOGISYS Computer PS575XBK 575W ATX12V SLI Ready Power Supply

G.SKILL Ripjaws Series 4GB (2 x 2GB) 240-Pin DDR3 SDRAM DDR3 1600 (PC3 12800) Desktop Memory Model F3-12800CL9D-4GBRL

Seagate Barracuda LP ST32000542AS 2TB 5900 RPM SATA 3.0Gb/s 3.5" Hard Drive -Bare Drive

IMC IMC-black USB 2.0 Card Reader

ASUS EAH5550/G/DI/1GD3(LP) Radeon HD 5550 1GB 128-bit DDR3 PCI Express 2.1 x16 HDCP Ready Low Profile Video Card

ASUS P7P55D-E LX LGA 1156 Intel P55 SATA 6Gb/s USB 3.0 ATX Intel Motherboard

Intel Core i5-760 Lynnfield 2.8GHz LGA 1156 95W Quad-Core Desktop Processor BX80605I5760

My overall budget is in the range of $200-250 (lower the better).

Hope you guys would be able to help me here.

Thanks

EDIT 1: I don't know how to check if the Graphic card is compatible with my Motherboard or Processor (or whatever it needs to be compatible with) that is why I am asking you guys. I tried to read on some computer parts specific forums and found out that the Power supply wattage also matters.

I would love to have some suggestions that are specific to me and which are compatible with my current things. I do not want to build a complete new computer as I don't have money right now. Thanks