I would suggest getting a good overclocking motherboard such as Gigabyte S3/DS3 or Asus P5B series MB and a good aftermarket HSF such as Scythe Infinity or Tuniq Tower so that you have the ability to adjust the computer to your performance requirements.
Pros: sell the DG965WH on EBay, can OC to higher speed than stock E6600
Cons: experience the "joys" of overclocking
DG965WH & E6600 Option
Pros: higher stock performance, no issues with OC'ing, sell E6300 on EBay, 4MB vs 2MB cache for a minor performance increase
Cons: no ability to overclock (stuck at stock speed)
1) What other components (memory, GPU, PS, etc.) are part of the build?
2) What are your objectives with the build? Gaming? Office Apps? Photoshop? Audio/Video encoding? etc.
I am planning using this as a gaming rig, my good ol 3.06c is getting a little dated.
GFX: Will be a DX10 compliant card, really just waiting on the R600 to do a little comparing, but have been eyeballing the 8800GTS
PS: Antec Power Trio 650
Memory: 2GB Crucial Ballistix 4-4-4-12
The biggest thing I was concerned about was the larger, 4MB L2 cache on the E6600, but if its a minimal increase I may just pick up a new board and juice up the E6300 because i got it for next to nothing