The core 2 duo's/quads just don't have the single-core power required to handle current games.
If you want a comparison, my old Core 2 Duo e4600 gives a single core performance of 12 Gigaflops whereas my brother's i3 2100 gives performance of 65 Gigaflops. And we're talking about a Sandy Bridge i3 here.
Some of the high-end i7's benchmarks show them crack 154 Gigaflops of performance.
If not, what video card should i put in this thing to retire it.
If you're going to use it, a 750Ti is plenty. If not, I don't see any use in investing further in this old PC.