I need suggestions for a good graphics card for my pc...?

Even on medium settings games such as Fallout, or GOW are jittery. If you have an idea or a link for a graphics card for my pc, please let me know. Here are my specs. in case you have more suggestions to help me improve.

Operating System: Windows 7 Home Premium 64-bit (6.1, Build 7601) Service Pack 1
System Manufacturer: Hewlett-Packard
System Model: p6754y
BIOS: BIOS Date: 09/07/10 08:41:32 Ver: 6.04
Processor: AMD Athlon(tm) II X4 640 Processor (4 CPUs), ~3.0GHz
Memory: 8192MB RAM
Available OS Memory: 7936MB RAM Page File: 3241MB used, 12627MB available
DirectX Version: DirectX 11
DX Setup Parameters: Not found User DPI Setting: Using System DPI
System DPI Setting: 96 DPI (100 percent)
DWM DPI Scaling: Disabled DxDiag Version: 6.01.7601.17514 64bit Unicode

Display Devices
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Card name: ATI Radeon HD 4200
Manufacturer: ATI Technologies Inc.
Chip type: ATI display adapter (0x9710)
DAC type: Internal DAC(400MHz)
Display Memory: 3063 MB Dedicated Memory: 252 MB Shared Memory: 2811 MB Current Mode: 1600 x 900 (32 bit) (60Hz)
Monitor Name: HP S2031 Series Wide LCD Monitor Native Mode: 1600 x 900(p) (60.000Hz)
Output Type: DVI
It can support PCI Express x16, but my power supply is probably insufficient because it's only 250w. I want to use it as a gaming PC. Would it be worthwhile to upgrade my power supply and buy a high-end card or could I get the results I want using a cheaper card that requires less power? I don't have to have the best, but I would like a more seamless gaming experience on my PC.