The E6600 is for sure worth getting as it is better than the X2 6000 on average.
In that price range I suggest this type of build.
CPU: E6600 (Q6600 after prices go down in July)
Mobo: MSI P6N Plat/ 680i board
Ram: 2Gb G.skill HZ's DDR2-800
Video: EVGA 8800GTX
Case: Gigabyte Aurora or NZXT Zero
PSU: Corsair 520HX/620HX