You will be dissapointed with Dual, Quad or 8 way CPU performance as a multi-interface system.
Multi-threaded, multi-cpu aware OS's and applications are really geared towards serving server style applicaitons.
What you are looking for is user interface responsiveness of 100% dedication to user interface requests as well as 100% responsiveness for server interface requests.
Doesn't quite end up working this way - a server served thread can be in a wait state as it gets in line to get it's slice of CPU - and all this happens without any real impact on responsiveness, considering all the latencies involved in a server serving environment.
Transactions still get serverd and the "user" whether a person or application doesn't really notice this latency.
A user on the console interacting with the system sees these wait states as pauses - something we as users are not prepared to deal with.
Remember, server OS's are actually architected to serve server threads of equal priority before user interface threads of equale priority (even in a multi CPU environment - it is still really just a really well managed menuing and scheduling system) while workstation OS"s are architected to serve user interface threads of equal priority before server threads of the same priority.
This is why Workstations have better user interface response that servers..
Been there done that - got the T-Shirt - two HT systems will server you much better than one gargantuan sytem