Linux is nice, but you had better be comfortable with using the command line, as most servers that I have run through Linux don't use a GUI.
Server 2003/2008 is nice for stability, but is often out of range for most because of price.
I really think XP is your best option if you are not comfortable with command line oriented programs. Also, have you ever used Linux? If not, you might want to steer clear of it, anyway, as you don't want something going wrong that you don't know how to fix quickly. Put Linux on one of your other computers to get used to it first, then load it onto your server.
Intel Core 2 Quad Q6600 @ 3.2GHz
4GB Mushkin DDR2 1000
Kingwin Mach 1 1000 watt Power Supply
Asus 4870 x2 graphics card
Gigabyte GA-EP45-UD3P motherboard
Cooler Master HAF 932 case
Samsung SyncMaster T240
3DMark Vantage:
X7241 (all settings maxed)
CPU: 11207/GPU: 7109