The i3 is a much better processor, and actually is sufficient for your needs-- I have a system built with an i3 (and HD 6870) and I can play Crysis 2 on high with ease. In fact, I just was prior to reading this topic.
Look here: http://www.tomshardware.com/reviews/gaming-graphics-card-review,3107-7.html
If you want to put anything on high settings that was made in the last year, or something intensive from the past two years, you want to stick to generally the top six or seven positions in either ATI or nVidia. As an added note, go with ATi on a budget, as they really do give more value at any point below the $200 range.
Ideally, for what you want, I'd recommend a Core i3 2100 and a GTX 460/HD 6850 or better. You -could- go a bit cheaper with the graphics card, but you don't get nearly as good of value for your money.