<< Windows 2000 Professional >>
W2K PRO will route so-so. It also has ICS (correct me someone, if I'm fubar on this), which means it can share the Internet connection OK.
Now, SECURITY for your setup is a different matter.
If you're gonna spring for W2K Pro, tho, you might think about springing for a wireless broadband router instead. Prices are coming down, and it would do the following:
* NAT firewall
* Wireless connectivity
* 10/100 switching
* DHCP, blah bah
The reason: Your W2K Pro box will be advertised WIDE-OPEN on the Web the entire time that the connection is live (basically: all the time). Plus, the load of routing the traffic will dog that machine something fierce.
A standalone router is a good thing. In a nutshell: why let a device that costs anywhere between $900~$1500 (give or take) get bogged down trying to perform a task it SUCKS at, when you could put in a device that costs between $200~$300 and KICKS ASS at that task?
JUST my $.02