This would be better suited in the systems category with "I want to buy a gaming PC" as the title. Also you should posted with the recommended format, so if you do not give more information the responses will not be 100%. But anyway...
The Core i7 is not a gaming processor in anyway. Go with a AMD Phenom II X4 955, this will save you money and AM3 has sockets coming to it so it is a good choice. Make sure you get a good quality PSU, 550W may be a bit restrictive for upgrades so I'd say get a 650.
Now go for a HD 5850 or a GTX 460 if you can't afford the 5850.