I'm wanting to buy a new graphics card for my computer but I don't know which one I should get! Also, how do I pull up my computer's information? Like graphics card and such?

My current computer stats:

Operating System
MS Windows 7 Home Premium 64-bit
CPU
AMD Athlon II X2 255 34 °C
Regor 45nm Technology
RAM
4.00 GB Dual-Channel DDR3 @ 669MHz (9-9-9-24)
Motherboard
ASUSTeK Computer INC. M4A78LT-M-LE (AM3) 45 °C
Graphics
DELL SE198WFP (1440x900@60Hz)
ATI Radeon HD 5400 Series
Hard Drives
488GB Hitachi Hitachi HDS721050CLA362 ATA Device (SATA) 35 °C
Optical Drives
Optiarc DVD RW AD-7260S ATA Device
DTSoftBusCd00
Audio
ATI High Definition Audio Device