Buying older tech has always been a good way of getting better FPS but it does come at the cost of the more advanced features.
The difference these days is that games have stagnated to the point where for a budget gamer it makes no difference really.
You don't need the latest cards from either company to comfortably play any game today at reasonable settings. We don't all need every setting on high
You also don't actually need DDR3 either. My latest Rig is an AM2+ board because i can reuse my DDR2 and save £100. I don't loose much performance and the whole thing is a lot cheaper. I also thanks to the above points about GPU's don't feel the need to get a new GPU for this build.
All of which means better MoBo,PSU,Case and Cooler. All in im getting a new PC for half the cost of the last one and it will be quite a bit faster as well.