Intel 3.1 GHz Core i3 2100 or AMD 3.6 GHz AM3+ FX4100

Overall performance wise, the i3 2100 is a better performer but cannot be overclocked. The FX-4100 is not bad and can be overclocked.

It really depends on each situation. The FX-4100 is almost a quad core, its 2 modules each giving about 180% of a dual core, while the i3-2100 is a dual core with SMT making it effectivley a quad core, but only about 20% effective to that point and most games do not benefit from the SMT in any way.

If he goes the AMD route, it would be better if he bought a Phenom II X4 or X6, as they will perform better than the FX-4100 and overclock as well.

Read this tom's article which addresses that exact question.
http://www.tomshardware.com/reviews/fx-4100-core-i3-210...

The conclusion is 2100 seems to be generally better.
I also note that the 2120 is priced less than the 2100 on the egg.

The 2100 can be overclocked a bit, but don't count on much.

IMHO i3-2100 is the best cost efficient cpu atm.

If he wants to overclock and stick with intel he could buy the last generation I3 540. It can easily go up to 4.0 GHZ.

No point of the last gen, when the next gen performs much better at stock clocks^^

don't forget that he also needs to account for a proper motherboard, both Intel and AMD offer similar MB with similar features but Intel may be slightly more expensive. If there will be no overclocking involved then Intel would be the way to go, if tweaking and OCing peaks his interest AMD would be my choice.

Thanks guys

He has bought Core i3 2100 and MSI 6850 Cyclone Edition gpu.