boht the xeon and opteron form great workstations.
here's some benches
http://www.gamepc.com/labs/view_content ... n36&page=4
use google - best friend for information
If you're going xeon, get noconas, if opteron, try to go for 244s or faster - that way you can use memory at ddr400 speeds instead of ddr333.
if you wanna go really
tiny, there's the dual via mini itx
For overclocking, it's tough to oc dual cpu systems, opterons especially seem resistant to it.
Gains of a dual cpu system - multitasking, and and multithreaded apps. Dont expect any performance increases when running single threaded apps (games, old software, some complex algorithmic stuff that can't be broken etc.) by itself.