The only scientific way of working it out is to DIY.
It all depends on the power of the box in question and the amount of RAM it has.
There are official figures in the documentation which serve as a starting point, but the only way to come up with an accurate figure is to pilot a server and monitor memory usage across different levels of user.
For example, it's one thing to say that Microsoft Word uses X amount of RAM, but that needs to be combined with the size and number of files a user might have open. Applications such as Internet Explorer can be dreadful memory hogs - and leaky to boot.
Everyone has their own rule of thumb, so here's mine;
128Mb for the O/S (Terminal Server)
64Mb for MetaFrame (XP needs this amount, but 1.x is significantly less).
32Mb per user.
IME, the processors make very little difference. Of course, there's a noticable improvement when using 1Ghz + over 200Mhz, so generally I get the fastest I can afford.
Scalability-wise, 2 processor boxes still give a better cost/performance ratio than 4 processor boxes. I'd recommend 2 dual-proc machines over a single quad any day - this could leave enough over in your budget for load-balancing, so you'd have a quasi fail-safe set-up.
I've had 100 users working comfortably on a single quad Xeon 500 server with 2Gb RAM. This was just a test to see if it could be done, though - not a live production server. The max I've had on a dual box is 75. RAM really is the key here - and since it's so cheap, max out those servers!
I hope this helps