It all depends on what your line speed is, for instance if you're doing this on say a 3.5M upstream than whats the point of having a computer that can process 29 times more then that in the same time.
Just think in terms of large CPU cache and ram i'd say 1 to 2MB/GB-cache/ram, a xeon would be pointless the EE P4's are virtualy the same thing only cheaper.
Don't use windows IIS, its jank behind jank, use apache.. I would recommend on gentoo.
for 50-60 users at a time (by that I mean initiating a request or receiving a response at the same time) with a low latency I wouldnt expect to pay more than 600 maybe 700 tops on CPU/RAM/MOBO
Of course let me add what exactly is this for, what do you expect from this server? if you have just a typical 128/256k broadband upstream then forget it, you'll handle 2-4 people at a time at 56k speeds, if this is just for friends or something local then i'd say use the 600mhz system, just be sure to install the right kernel, I could tell you if you told me what the cpu in that system is.