Without the 'K' on the i5-2500 means no overclocking other than baseclk. Baseclk is dangerous to OC with sandy bridge. I would not want the 10% OC. If you do OC get the Intel Core i5-2500K and overclock via multiplier.
GTX 590 is huge. Not the best price performer, but a great performer. Make sure your room has good cooling. grin.
I think your disk is way too small for gaming if you use steam. (I like steam, no CDs to lose). Go for 1.5TB or 2TB.
I would personally go for more than 4GB of memory with this system because this system should be strong for 3+ years, so you will end up adding it at some point. 6GB or 8GB.