Nt 4.0 or Xp not 2k from experience, also if your os sees 2 proc then the game uses 2 proc regardless of design of programming.It will use the background geometry (being dedicate) threads seperate on each proc.Run a server and check your task manager look at proc usage it will use both on any and all games( fact ). Yeold will prob add to this has he also has alot of exp in this area has well.
The worlds shortest book...... "Things I cant buy" by Bill Gates.
Right click taskbar after running a server, click on task manager look at performance.You will see 2 windows for each proc see what % each on is useing has more people join server watch it rise, I have done this so many times, monitoring servers proc.Click on networking check the mbits not very accur though.
Look at taskmanager before game,launch game ,play it then click alt\tab look at taskmanager again check proc usage of both windows, its a fact if the os uses both so does the programs FACT.
The worlds shortest book...... "Things I cant buy" by Bill Gates.
Q3 runs fine on either 2K or NT4....but if you take the time to try both and benchmark the server, gather opinions, you'll find NT 4 Server will give the best performance. It runs the leanest (shouldn't be a surprise that 2K is more bloated and hogs more resources for itself).
For LAN parties and co-locating at ISP's....NT 4 with SP6a is still performance king for me.
Make sure you get a Server OS, not a desktop OS like NT 4 Workstation, or 2K Professional, or XP. Desktop OS's are designed to work well with "up to 10 concurrent connections", servers obviously designed to handle more than that.
And don't be cheap with your NIC...get a 3COM 905, or better yet, a 990xp (best of all), or an Intel Pro.
As to the multi proc arguement....there's two approaches. Some games (Q3 engine for example) will see multi CPUs...and truely split the load.....it runs really nice on dual procs. I even built a budget LAN server for someone with dual Celery 500's and coudn't believe how well it ran....35 % CPU utilization running 2x Q3 servers with over 50 peeps at a LAN.
Other games that don't take advantage of dual CPUs, like UT engine based games for example....well, the game itself won't split the load between the CPUs....but the OS has already split the load of running itself (the OS) between the CPUs.....so each CPU is already running lighter, since each individual CPU is only running about half of the OS's duties.....so it has a lot more free cycles to take on the game.
In other words...take a single PIII 900 box, and a dual PIII 900 box...all other things equal...on the dual PIII box...Q3 will REALLY perform MUCH better...since it natively supports multi procs. But UT, even though it only supports single procs...will still run better because each proc on the dual box is already running lighter, since the OS distributed it's jobs over both procs.
I'm rebuilding our public gaming server this month with an older dual PIII box I got back from being our clans LAN server. I have access to any OS that I want....and NT 4 Server is the one going back on.
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Yeold will prob add to this has he also has alot of exp in this area has well. 
