Minimum SB CPU to Keep Up With a 560 Ti

Might could go less but I'd recommend this i3. Pretty cheap.
http://www.newegg.com/Product/Product.aspx?Item=N82E16819115078

A simple i3 will be enough, don't subestimate it's power. Hell, my own i3 can feed both my 470's with little to no bottleneck, and i play @ 2048x1152 (i know it is heavily OC'd, but a single 470 is more powerful than a regular 560 Ti, not to mention two.)

My i3 2120 has no problems at 3.1GHz (I matched it to a 2100) with my 7970. I honestly can't tell a difference in gaming between the i3 and my i5 at 5GHz.

For the extra 5 bucks, its worth it to grab the 2120 though, mine does 3.5GHz stable.
http://www.newegg.com/Product/Product.aspx?Item=N82E16819115077

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.

A good choice. If you want to upgrade later... you'll gain roughly 2.5x the processing power w/a 3770k w/moderate OCs vs a 2100/2120 (~40 Gflops vs 100+). While it isn't all cut and dry for gaming, it will certainly give you ~1 Ghz more w/a realistic OC, plus double the cores/threads... not to mention architectural improvements. Again a 2120 is a great chip. My old Q6600 would (could) be put to shame at times w/a new i3. The 1156 i3s would make it look like a dog w/the massive overclocks that were possible.

1155 may be at the 'end of the road' for new processors, but it will be viable for many, many years. You still have a great uograde path if you want to put some more cash into it. Your 2120 will likely also hold a bit of it's value to offset an upgrade cost.

The Celerons are good (don't let the Celeron name put you off... they have little in common w/the old gen chips). They tend to have a lower clock speed and none have HT. They are also quite cheap. For a placeholder for short-term use, if you know you're upgrading really soon, they're great. A lot of folks that did Z77 builds bought them to run their systems while they waiting for IB to release. They just aren't worth it IMO for long-term use, if you know you're gaming and want to hold off an upgrade. The large clock speed difference (mid to upper 2 Ghz range) vs over 3 ghz, not to mention HT put a hurting on them. They're that price for a reason.

My i3 2120 (recently sold) easily outperformed my previous C2D E8500 @ 4.0Ghz by a noticeable margin.

I had the i3 2120 + 560Ti setup running for several months and could game anything, but mainly Skyrim. I was getting 50-60FPS on Ultra/High with 4xAA and at 1080p.

Go for it, you won't be disappointed!

1155 will be dead here not too long, so don't base a decision upon that. SB is still pretty powerful, and an i3 should be plenty. Keep in mind that lower resolutions usually rely on CPU more, so you may have issues if you are gaming too far below 1920x1080.