As long as your motherboard has a PCI-Express 16x slot, you can use ANY PCI-Express card from any generation. PCI-Ex is backwards compatible.
Now, don't expect reaching very high frame rates at high resolution. Yes, the GPU will be bottlenecked by your CPU. I guess you can stay at 720p without problems. The bottlenecking depends on the games, some use more CPU resources, others less. If you manage to OC your CPU, you can reduce the bottlenecks, at your own risk.