Sandy Bridge is currently (3/2012) the best processor option on the market for all around use. They are just fast at everything, generate low heat and low power consumption. The i5-2310 is one of the cheapest Quad Core options. If you compare the different models, you will find that there is only about a 3.5% performance difference between them, or basically you won't really be able to tell unless you run benchmarks instead of actual programs.
Comparisons: i5-2300 - 2.8Ghz -3.5% slower than 2310 i5-2320 - 3.0Ghz +3.4% faster than 2310 i5-2400 - 3.1Ghz +6.8% faster than 2310
So with them being that close, I'd say just buy what's on sale that week. They all have the same basic features and excellent performance. When I bought, it was the 2310 and I have not been disappointed in the slightest. It seems easily twice as fast at everything than my Core 2 Quad.
I have yet to find a game that i cannot run with this CPU. Looking forward to upgrading to the i7 later this year! If you are looking for powerful CPU at a descent price, i would highly recommend grabbing this CPU as it can run any game if you have a good GPU.
I know Intel CPU's are pricier but under the right circumstances they are unbelievable performers. I could not believe how far I pushed the envelope on this CPU and it never missed a beat. In a properly air-cooled case, it cruises all day at 3.5 GHz with temperatures below 50 degrees (44-47). I've never had an INTEL CPU fail on me in decades; and I've build more rigs than I can possibly remember.
One of my hobbies is to build all of our PCs. I have built about 10 in the last 5-6 years. I built one using the Intel Atom and another using an i3 CPU. The i5 is mush better and faster. The Atom (on a Zotac motherboard) did not have enough horsepower for BluRay videos and 5.1 audio for a home theatre PC. The i3 was better but for a little more money the i5 is a better deal. The fan supplied with the CPU was easy to install and is very quiet.