I game happily on high settings at 1440*900 on Fallout 3:NV and COD:MW2 etc using a Radeon HD4650 with DDR3, something like this but mine's a PCI-E one from Sapphire and with 512MB DDR3 RAM (processor is an E5300 at stock, 2GB system RAM).
Not sure how much difference it makes 512MB vs 1GB, what I bought was just a slower clocked 4670. I think going for the DDR3 does make a significant difference, IIRC from the reviews I read at the time.
Not sure you'd be able to tell the difference buying the higher clock speed of a HD4670 (I managed to overclock mine up to the same speed with the Overdrive utility
but it made no difference to anything but the power consumption and fan noise).YMMV - don't shoot!