Ok, first off you don't want to go Core2Duo or Core2Quad because the socket they use will not offer you any upgrade path. The E8600 is very good for gaming but is an extremely overpriced CPU and is really only good if you are an overclocker/enthusiast.
If you want to go intel I suggest going for the i5-750, or one of the i3s if you are on a tight budget. i5-750 is one of the best price/performance CPUs on the market.
As for graphics stay away from the GTX285, that GPU will cost you an extra $100 more than its worth. Go with something from ATI like the 4870 you had picked out or something newer from the 5xxx series (5770 is priced similar to the 4870 with similar performance and better tech/efficiency).
The i5 750 is better than theE8600 in everything, including gaming. when games only use 2 cores the i5 750 will boost the clockspeed of those cores to 3.2 GHz, which is only 100 MHz off from the E8600, and it is faster clock for clock. When using more than two cores the i5 750 will boost its clock speed to 2.8 GHz and the E8600 doesn't have more than two cores.
Unless your memory in your current system is DDR3 then you will also need new memory if you go with the i5 750 and p55 motherboard. If you do not have the money for all of that then you could consider getting a Core 2 Quad off Ebay for a decent price, but I wouldn't buy one new.