Unless you happen to want to play a CPU intensive game NOW, I would suggest putting off the CPU for later when it is more necessary since for 1GPU setups, an OC'd Ph2X4 is still enough for 90% of the time, and just OC'ing your current Ph2X4, a nice cooler should be able to get it well past 4GHz.
Though, if you want to buy the CPU/mobo now, the i5 is plenty.
The 660Ti is nV's equivalent. AMD drivers are about as good as nV drivers nowadays for most things. Performance wise, AnandTech has a decent list of benchmarks, pick and choose what games you want.
http://www.anandtech.com/bench/Product/550?vs=647