What uses do you have in mind for the system you end up buying?

I also need to clarify one thing here, as I'm a bit rusty on AMD's method for naming Opteron chips. The Opteron systems would be 2 CPUs with either quad or dual cores each? And the Intel system is only a single quad core CPU?

Because if that's the case, I'd definitely have to give the nod to the quad core Opteron setup. Having two quad core CPUs would give you a massive amount of processing power to hold you over for quite a while to come as the world moves more and more into parallel computing. Even if each core is slightly slower than the other options, you have considerably more combined processing power.

If it's just one CPU in all three situations, I'd probably give the nod to the Intel chip for most things.

That is assuming I am reading what you said correctly, and price differences are negligible and all other specs are more or less identical.