That depends on what OS.
Win9x/WinME only supports a single processor.
Win NT/Win2k supports multiple processors in an SMP (Symmetrical Multi-Processing) system. That means that all tasks are spread evenly across all the processors. However, you can set certain programs to run on specific CPU's if you like.
Win2k runs pretty well in a multi-proc setup. I run a dual Celeron 366@500 as my server/proxy box at home. I also use it to host Q3TA, Diablo II, and things like that with no problems. Still, how well a program can take advantage of multiple CPU's is dependent on how it's written. Although the OS is able to multi-task, unless the application is written with multi-threading in mind, the OS can't do much with it. A good multi-threaded application will be able to take much more advantage of a multi-proc machine. The only game I know with any sort of decent multi-threading is Quake 3... and it's gains are minimal speed wise. I find the big difference is for servers, an SMP box is much more tolerant of processing hits than a single proc box. What I mean is, the OS is spread over all the CPU's, so things that hit the OS for CPU time don't hurt you as badly. For example, if somebody is streaming an MP3 off your server while you are hosting a Q3 game, a multi-proc box won't be as likely to show a slowdown as a single proc box would.
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Lephturn - Chief Trainer
A member of The Flying Pigs
http://www.flyingpigs.com "A pig is a jolly companion, Boar, sow, barrow, or gilt --
A pig is a pal, who'll boost your morale, Though mountains may topple and tilt.
When they've blackballed, bamboozled, and burned you, When they've turned on you, Tory and Whig,
Though you may be thrown over by Tabby and Rover, You'll never go wrong with a pig, a pig,
You'll never go wrong with a pig!" -- Thomas Pynchon, "Gravity's Rainbow"
