At a lower resolution like 1400x900 you certainly do not need anything like the GTX 470. It would be overkill for your needs (and therefore a waste of money). With your resolution, you could play those games with an ATI 5750 or ATI 5770 and do pretty darn well. Not to mention it would cost about half (or less) what the GTX 470 does.
Getting anything faster than an ATI 5770 is probably a waste considering your processor.
Make sure your processor has a PCI-E 16x expansion slot. And also, you'll probably want to look at the specs on the Power Supply in your computer to make sure it's up to snuff as well. If you're not sure, take the side panel off your PC. There's usually a sticker/label on the power supply itself that gives a brand, model number, and important details (max watt output, Amps on the +12v rail).