750 as it's a quad core as opposed to the 661 which is dual. The architecture and turbo boost is more than enough to cope with games that previously would have run better on a dual.
If you need integrated graphics though, it'll have to be 661
1- For a gaming rig you need at least 4GB
2- The best option is the i5-750
3- If your money is enough, try to get the i7-860, is a very good processor with the a very similar performance of the i7-920
I5-750. I will never recommend the i5-661 until the price drops significantly. You have a discrete GPU, so you don't need the integrated GPU. The 750 is a true quad core as opposed to the 661 dual core with hyper-threading (which is really a gimmick as far as gaming goes). The 750 will have far superior performance for the same price. No reason to ever get the 661 IMHO- way overpriced and quad core is the way to go these days.
You can get 1333 for $80 if you find a deal as well (like the top one in the link I posted is 1600 for $80 after MIR) but generally its going to be ~$100 +/- $10
Well, in newegg.com you can find a good 4GB kit (2x2GB) like THIS DDR3 1600 at $ 114,99 with free shipping, now, you need check in the store of your country the prices.
Also THIS DDR3 1333 at the same price that the DDR3 1600