The only real minimum requirement that WinGate has is IE4 and above. If the computer is good enough to run the OS, it's certainly good enough (spec wise) to run WinGate.
Keep in mind that it will also depend on what your users are going to be doing, and whether you want to run AntiVirus on that machine as well, since AV scanning takes a lot of CPU. We have customer sites where there are well over 250 concurrent users hitting WinGate all day long on a P4 3GHz machine without any problems.
If you want to run AV though, you can divide that by between 3 and 6, again depending on what your users are doing.
Bear in mind also this is concurrent users, not seats. Normally not everyone is using WinGate all at the same time.