At Be, we don't have a specific recommendation for platform. Instead, we recommend that customers use either an Intel- or a PowerPC-based system, whichever you own today, or whichever you prefer.
If you don't have a platform preference, however, and are planning to buy a new system for the BeOS, frankly, the price/performance ratio seems more compelling on the Intel Architecture side. Just take a look at our Ming Specials page for an example of how inexpensive high-performance hardware can be:
In particular, when buying a new system, the number of options and pricing for multiprocessor hardware is simply greater on the Intel side of the fence. The BeOS really loves processors, and will be happier the more you give it...though it runs great on single processor systems, too.