Processor speed should not be that much of an issue (a P-60 is fast enough for very basic operation) unless you're planning on doing real-time audio processing, video stuff, etc. I'd personally recommend a Pentium 133 as a minimum.
If you're doing lots of graphics work or playing back video, a faster processor will certainly deliver a better user experience.
If you're doing things like real-time synthesis using ObjektSynth, you'll probably want a 1.2GHz Athlon to get a decent amount of polyphony.
For some OpenGL apps, however, the fastest processors currently available might not be fast enough. (Official support for hardware accelerated OpenGL doesn't exist in BeOS R5; it's all handled in software. (OpenGL support was going through a re-write at Be, and sadly, it didn't get finished before Palm bought most of Be's IP.) For example, GLTron has been ported to BeOS, but you'd probably need a quad 2GHz Athlon MP system to get decent performance!
I've run BeOS R4.5 on a 200MHz Pentium MMX-based machine and performance was great. Later, I ran R5 on a 350MHz AMD K6/2. Digital audio performance--with real-time effects—and video playback was smooth as silk. When I switched to a 550MHz K6/2, I couldn't tell any improvement in performance except in cases in which I was putting a pretty heavy load on the system.
I have run BeOS R5 on a 1.2GHz Athlon system and it SCREAMED. Incredible performance!
As far as amount of RAM you need, that depends on several factors as well. If you have very little disk space or a very slow hard drive and want to minimize the amount of virtual memory, you'll obviously want more RAM. If you're working with enormous multimedia files, you'll want more RAM. For basic functionality, you need surprisingly little. 32MB should be sufficient for basic functionality, 64MB should be plenty if all you want to do is surf the 'net, send and receive e-mail, and do some light gaming. 128MB is plenty for just about everything unless you're working with really, really huge audio or video files, in which case you might need more.
Sorry to be so vague, but it really depends on what you'll be doing.
With CPU, memory, and system board prices being as ridiculously low as they are, you shouldn't have much trouble getting the power to do whatever you need to do with BeOS.
Everything I have done under BeOS requires far less CPU power and system RAM than under Windows or even Linux or Solaris. (Although Linux, using a light modular 2.4 kernel with latency patches and without some godawful desktop environment to bog it down, can give it quite a run for the money.)
