Dual opterons could be quieter at the same throughput due to spreading out the heat/ noise
even if they use more power
power supply would depend on the motherboard, video card, etc.
one x2 would be cheeper than two opteron setup but no way to go 2 x2 later
what's your application?
how much memory do need?
2 opterons have twice the memory bndwidth of one x2 ( but this matters in only a few apps- but it matters if yours is one of them)
one x2 can quickly access all of installed memory wheras it would be split between two optrons with half of the memory requireing a longer access (again this matters only with a few apps and you can buy more memory to fix the problem- if it is a problem)