Upgrading to HD5670?

Hello, I am currently looking to upgrade my graphics card, basically wanting the best I can get for my current PC.
I have a Dell Inspiron 537 . I have contacted Dell support and they have recommended an AMD Radeon 5670 1GB ( this or this ) as the best compatible card (albeit after saying my computer could only handle a 512mb card). Their 537 product site states Radeon HD 3450 256MB ( this or this ) as the top compatible card.
If necessary I will also upgrade other parts, such as my RAM or Power Supply, but I am looking to keep it cheap.
Thanks, any help is greatly appreciated.

APPROXIMATE PURCHASE DATE: ASAP BUDGET RANGE: Under $200 for the card

USAGE FROM MOST TO LEAST IMPORTANT: Looking to play newer games, current card is stuttering on lowest settings on Fallout: New Vegas.

CURRENT GPU AND POWER SUPPLY: Intel G41 Express Chipset and 300w power supply

OTHER RELEVANT SYSTEM SPECS: Intel Dual-Core E5200 @ 2.5GHz, Dell 0U880P A00 mobo, 4gb RAM. 2 PCI slots, 1 PCI-Expressx1 slot, and 1 PCI-Expressx16 slot

PREFERRED WEBSITE(S) FOR PARTS: Would prefer a store rather than a website, but any link is helpful, if only for comparisons. COUNTRY OF ORIGIN: Canada

PARTS PREFERENCES: I have had good experience with Nvidia cards in the past, but I have no real preference.

OVERCLOCKING: Probably not SLI OR CROSSFIRE: Whichever, have used SLI before, not crossfire

MONITOR RESOLUTION: 1600x900

ADDITIONAL COMMENTS: I don't need the best of the best, but would like to play games with a moderate amount of detail and smoothness. I have previously owned an Nvidia 8800 gt which worked great for my needs, even playing crysis at (nearly) full at the time.

Thanks again, this community always has the best advice!