It looks like your on a budget but if you can afford the I-5 2500K it is a screaming fast CPU that Tom's Hardware recommends as the best bang for your buck. Your system will be much faster than the AMD system and more "future proofed" if there is such a thing. There is only about a $60.00 difference in price. The 2500K hovers around the $210 cost. To give you an idea of the speed, it beats the I-7 990 (a $1000 CPU) in many benchmarks. If you game at all speed will be very important.
The ATI 6870 is a great choice. The ATI 6950 1 GB is about $50 more and will give you a lot more speed if you can afford it.