ahh, heck, you don't want that useless junky opteron. Just hand it over, and the MB too.
edit: honestly, I just went through the justification over with board etc to buy, and I can't see a single reason to buy the Core 2's at this point over a 939 chip. (I happen to have 2GB of DDR sitting around, hence the 939 vs AM2 choice).
Here's the reasoning: Socket F is coming in 6-9 months. It is the designated socket for K8L. Core 2's gain you almost nothing in real world gaming over an AMD choice and won't for at least 6, and probably more than 12 months - it's the GPU, stupid (to paraphrase).
If you don't have a high end rig, You can pick up an AMD 64 X2 3800+ w/ MB @ Fry's for about $200. Core 2's will cost you at least $450 for the combo (cheapest I've seen combo'd is $500). You can pick up a AMD 64 3800+ (or better single core) for a mere $130 last week.
Unless you're into photo/video editing with multi-threaded applications, Core 2 buys you nothing at this stage over either AMD chip mentioned above. By the time it will make a difference, AMD's answer to Conroe will be out, and from the initial looks of the specs, it looks good - you didn't think IBM bet blindly on AMD for its server line, did you?
Also, if money's burning a hole in your pocket, a $500 GPU or SLI rig would do more to improve your game than anything else at this point.