Well, you can never have too much disk space...
Capacities up to 80GB have been tested. The only thing you have to watch for is the 80GB drives specifically are "pushing the envelope" with the memory available on the empeg (especially the Mark1 and Mark2s). You see this especially with the drive builder software and when you do file system checks. But they do work.
The empeg does not need a high speed drive. The cheaper 4500RPM drives are just fine. In fact, better because they generate less heat.
Meanwhile, make sure you check out
the drive upgrade FAQ for step by step, detailed instructions on how to do the upgrade.
As for price (re-reading your original post, and seeing that I misread...), I always use the old Googlegear, now
ZipZoomFly.com. I have been extremely happy with their service and with their prices. Pricewatch and C|Net also give good pricing guidelines on what is out there.