Hmm gameserver has nothing to do with clients. AS the matter of fact, game server dont even runs the game. It just do the job of linking clients and distributing data. Thats why gameserve is intensive in CPU and RAM.
So in summary, what OS gameserver runs on has nothing to do with what OS clients on. If you runs HL game server (any CS, DOD...) 700 is enough . For RAM i recommend 256mb , a small HD(1.2gig is enough) and a good NIC. You good to go.
I runs a CS server(16ppl) myself(Piii700mhz, 398MB), i found Linux is the only way to go for gameserver. It has the best resource management anyway. Beside you dont need keyboard, mouse and monitor if you're familiar with linux. You can setup linux thro network or comile the source on your comp then put the HD in and fire up (remeber to compile sshd and setup the nic).
The avg CPU and Ram %usage with HL server is 40% and 20%
***note: that is with Ping booster***
Ans AFAIK your setup can host 16ppl server for MOHAA, UT2k3 and 12ppl BF1942.
Hope that helps..have any doubts just ask
MameXP,
PS. dual cpus WONT help you unless you host more than 1 gameserver.