I'm also setting up a Workflow server for my corporation and needed some specs...  I pinged my instructor for suggestions about the requirments for the Workflow server...here is what I found out that I would need for my environment:

Windows 2003 64-bit
16 GB of memory
4 CPU
SQL 2005 64 bit (can be local or remote). Due to the additional SQL Analysis security requirements, local SQL is far easier to implement.
w/ Full Text Search
w/ SQL Analysis services

He wasn't sure about any information regarding the production SQL load, but for a big implementation, he recommended dedicating 4 SCSI/SAS drives in a RAID 10 array.

My environment is running about 1,600 nodes, so his specifications were based off of that.

Hope it helps!

Tom