I have a pentium dual core system using an intel G645 processor at 2.9GHz (sandy bridge) and am thinking of upgrading from HD graphics to a radeon 7850. Would my processor be too weak or is it enough? I play games at 1080p by the way. Thanks
More aboutwould bottleneck
Look up the recommended specs of the games you play. It will say dual or quad core. Most games are heavily GPU dependant. However i5's not only have 2 more cores, but double the cache (6mb vs. 3). Sometimes that's important.
Your GPU should be fine for your CPU. Do not expect any major bottlenecks, but you will not be able to max out a lot of modern games. On medium settings, you should do fine.