yeah, the Opterons shred the Xeons.
a 200 series Opteron will really beat 2 Xeons (they can do 2 tasks at a time by themselves)
or, an 800 series Opteron (which can process 8 tasks at a time by itself) will not only run circles around the Xeons, but will leave wake turbulence that will rip all the parts out of it.
not only that, but now they have dual core versions of these beasts!
it is possible to get a quad motherboard so you can get 4 of them too. 4 dual core 800 series Opterons = 64 threads at a time. a dual Xeon system will be snapped like a twig under the power of them.
the downsides: they have to use ECC RAM. it's a little bit bit slower (more than made up by the raw CPU power), but more stable. there are not a lot of motherboards that support the 800 series Opterons, and when you start getting into multi 800 series Opterons, they can end up costing more than $10,000.
if you can find a motherboard that has AGP, supports a single 800 series Opteron, I'd go for something like an 844 or 846 series Opteron (the 846 is $596 at www.atacom.com
without a cooler) then that'll givce you much more than the dual Xeons.
although the 200 series Opterons are not extremely expensive.
for $206, this is a dual socket 940 motherboard that has AGP, and supports the 200 series Opterons.
that, with 2 Opteron 248's
for $327 each ($654) will give you much more than the dual Xeons