From the Cognos Website, they recommend a Pentium III at 500MHz & 512MB Ram (256 for client) and 3.5GB HDD + 0.5GB for Temp and another 0.5GB for 3rd party programs.
FWIW, I have Admin imp & transformer on an PIII with only 256Ram and it's manageable. However, our new twin 2.4GHz Xeon server (2Gb RAM with ultra SCSI disks) zips along and makes Upfront acceptable for users.
HTH,
lex