The 8400 will prob clock to 4ghz stable. Games will benefit.
The q6600 will prob clock to 3.6ish. Encoding/decoding/photoshop etc will benefit, oh and benchmarks.
Depends what you want from your rig...
If you do get the 45nm just be careful with the voltage. 1.45v MAX, peeps treating them like 65nm and banging 1.55v through them just to see how they clock/bench soon discovered their once stable OC was unstable, needing more voltage for the same OC. Rapido degridation.