Its hard to say but for certain apps the dual 242's will be a lot faster then the fastest athlon xp (socket A), but for most games..etc the 242's will be slower due to the very low clock speed, even though they have a lot bigger cache...etc
Pro apps/64bit 242's faster, most (single threaded)games socket A athlon xp3200 will be faster.
It's difficult to make a comparison between a single processor XP and the dual processor Opterons. The differing clock rates complicate things even more.
What the Opteron set up would give you, thanks to the A64 architecture, is an on-die memory controllers, which provide absolutely massive internal bandwidth to the dual processors. Assuming that you can assign the process affinity to either processor, and thusly dump all of the "housekeeping" tasks on to one CPU, and run the single threaded app or game on the other, them I suspect that the Opteron set up would surprise and delight you. That on-die memory controller is astonishing.