I am actually not a developer, just a knowledgeable user (my information is accurate.) However, RunUO uses Overlapped IO to manage concurrent connections.
To increase performance, the number one influencer is RAM. I suggest 512Meg as a minimum, 1Gig or more prefered.
Regarding multiprocessor, I am running dual Xeon's in hyperthreaded mode. Whilst RunUO's core is single threaded, the operating system and whatever else (firewall, virus scanner, maybe a ftp server) is not in that thread. Multiprocessor pros/cons are somewhat debated around here, but I am on the pro side myself.