You have to give and take. If you want to have Core 2 Duo, you need to give up on AGP or docking or screen to reach $300 limit. Or, you can have a low end C2D such as T5600. I was saying you can install a T7600 because that is the last to support the 667MHZ FSB.
In fact the CPU differs quite small:
Intel Core i7 720QM @ 1.60GHz 3276
Intel Core2 Duo T7600 @ 2.33GHz 1309
Intel Core2 Duo T5600 @ 1.83GHz 1001
Intel Core Duo T2400 @ 1.83GHz 885
You won't feel the difference.