I would err on the lower end whenever you're talking about a Crusoe. I have an 800Mhz one in the Fujitsu Lifebook P2040 subnotebook and, though I love the thing, it is slow. It runs what I need it to though, and it's not too slow most of the time. It would probably be around P3 500-550 though some of the slowness could be attributed to the 4200rpm hard drive.
If you are planning on using the notebook for basic web surfing, email, office stuff, chatting, it will do. 🙂
Gaidin