Well, I certainly wouldn't do the i5-650. It's not worth it when the i5-750 is almost the same price.
And right now, there is no such thing as being CPU bottlenecked. The GPU is always going to be the bottleneck.
Have you considered AMD? If you get a X4 620 and Asus M4A79XTD EVO, you might be able to afford an HD 5870. And that would be a better gaming system.
Refer to this article. The i3 is on par with the lower AMD X4s (630), but priced more like the higher X4s (955). The i5-660 is on par with the X4 955, but priced like the i5. So really, I wouldn't go with either the i3 or i5. Go with a cheaper AMD build, and you may be able to improve the GPU, or other parts of the build.
I've checking, and the situation has changed, and at this moment I have the chance to change i5+4890 by i3+5850, expending only 25 Euros more in the second option than in the first. I want to be sure that I buy a system that allows me playing games for, let's say, 3 years, and I'm not sure if the i3 or the 4890 allow me this. What would you say?
The i3 is a entry level CPU. Lower CPU power. Plus with the i9's about to hit the market you may be better off waiting to see what it does to lower the prices on all the other chips. If you really want to find out the differences and all the details about the different CPU's you can do things like look them up on Wikipedia and there is also http://www.cpubenchmark.net/ for benchmark scores and many different CPU's.