2100/2120 are good choices if on a strict budget. They can put a C2Q to shame, unless it has a decent OC and then it'll still give it a run for it's money. If you're sticking w/a single 560i and always at 1080... you'd be alright most of the time. Another 560Ti in SLI will be useful most of the time, but there'll be times that minimum framerates will suffer. I'd definately take a 2120 over the 2100. If you're planning on having it awhile, every little bit will help. Recommended/minumum requirements are only going to go up and a few extra bucks now would be worth it if you're going on the cheap but want it to last as long as possible. OCing is very limited. A lot of it 'being enough' is going to depend on the games being played. Some games are just going to be pretty CPU dependent. There are games made years ago that'll run well under 60 fps at time due to being so CPU dependent, like GTA4. There'll be games made years from now it probably won't break a sweat on. It's just a crapshoot w/low to mid-range hardware.
If it's not needed *right* now, waiting for IB i3s in and around the fall might be worth the wait. They'll be 3.3/3.4 Ghz. Since OCing is very minimal, it'd be like getting an extra 200 mhz based on the new cores... assuming their clocks are the same. It may also cause some price cuts on the SB i3s.
If not limiting yourself to 1155... 1156 Clarkdale i3s OCed quite well and would cost about the same. They oughta perform better, based on their OCing ability, for gaming. They might be able to be found used on the cheap. I see some going for well under $100.