-It seems that finding some older Intel Mac Minis (they can be easily found for around $300ea) might be what you're looking for, but that wouldn't solve your need for XP.
-A single not-so-cheap $600 computer will probably
crush 2 separate $300 computers for performance. If you don't need separate computers, consider going that route.
-You might also consider looking for something a bit more lopsided. Get a clunker for $130 and a decent one for $470 and then distribute tasks accordingly.
-Make looking into the site license your first priority. If you don't have to blow $200 on two copies of Windows, that will
really make a difference for your hardware budget.
Otherwise you're going to be looking at things like
this* and
this* which, while they meet your stated needs, are probably not what you
really want. If you look hard enough, you could probably find people willing to pay YOU to take an old Pentium 4 off their hands**.
--Patrick
*assuming the links still work.
**not really. Sorry. But close.