Well dual cores are faster but not quite what people think, they do not actually double the processor power.
http://www.windowsitpro.com/Article/ArticleID/48520/48520.html
This coupled with RUNUO and Windows not being true 64 bit multi threaded and your pretty much stuck...
If your tight on money I would recommend the single dual core with a couple Raptor HD's (due to raw speed) and perhaps some high end ram on a good motherboard. It would be more than enough to run a small UO shard.
Dual core with dual processors would be nice for the future though since the next windows system is supposed to take more advantage of their power.
Perhaps a compromise...a dual socket motherboard with a single dual core installed to see if it performs to your liking, if it does not then you can always try the second processor.
Of course you could go the overkill route and buy a rack server 5U with a backplane for 4 processor cards.
I have a 20U and a 52U rack I'm working on filling up, 2-5U bays are already done but they are older units for crunching numbers for various diabolical projects. The next 5U rack will be for a game server of some sort. You can pick up the rack units at Electronic surplus stores brand new, still in the box/plastic for around $150.00 per 5U so its actually pretty cheap to put together.
Dev