450mhz should be fine. I put XP on my family's computer which runs at 400, and it does decent. 128mb of memory is gonna be your biggest bottleneck though. Since memory is dirt cheap right now, I'd suggest adding *at least* another stick of 128, or more than that if possible. On my own computer, it currently reports using 119mb of physical memory, and that's running nothing but Windows and its standard services, Apache server, and Winamp. So add lots more memory if you can.
The only 9x program I've ever had a problem with on XP is the BlackIce firewall, and even using the compatibility manager thingy it won't work. Microsoft has a net-based compatibility checker if you're unsure, so try
THIS to check your hardware and software.
Hope this helps.
---Jamin
