I m in the process of upgrading my old rig,will be getting an i3 2120 now and upgrading to i5 3550 in few months,4 gb ddr3 ram.I have 1440*900 res now which will be upgraded to 1080p along with the processor.The damn physx and amd's crappy drivers are making my decision tougher.
Get i5 3550.No need to buy i3 2100 and then upgrading to i5.Not worth the money spent.
As far as gaming is concerned under windows amd drivers are not crappy.There can be issues with some version of drivers which is also true in the case of nvidia.
Overall hd7950 is a better card for gaming.
One can't run physx on amd gpu.One can add a secondary nvidia card for physx processing with a primary amd card.
Have to use hacked/modded drivers for that.
There are other alternatives like
Havok and some other CPU based physics engines which would run irrespective of amd or nvidia gpu.
There are not that many good titles with good PhysX application/implementation(I don't call pre-rendered debris effects impressive).
The thing is Physx is not something that is that eye-catching in many games.
Mafia II is one good physx title.
At the end it is your choice.If physx is that important to you then take the gtx660ti dcII.