I5 750 or E8400 for gaming?

Why not go for one of the Core2Quad chips instead? They're a lot cheaper than the Core i5 when you take the cost of the motherboard into account, plus they're quite capable of supplying any current graphics card with all the data that it can handle. DDR3 memory isn't much different in price from DDR2 so maybe you should get the best. Of course though 8GB of DDR2 is better than 4GB of DDR3. Maybe you could use the money that you save to buy an HD5850 or HD5870 instead, since this will make a bigger difference to your framerate than a faster processor will. If I take my HD5870 out of my Core i7 920 rig and put in my daughter's Core2Quad Q6600 rig it still achieves a similar framerate.