There is not, its suprsing you cant considering xbox is not much more than a 733mhz pc anyway, probably that the custom graphics hardware isnt compatible.
The reason you need a faster CPU is that the XBox OS is a seriously stripped down verion of Windows 2000. ie, there's nothing running in the background when it starts up and it doesn't try and do 'clever' things that Redmond deem necessary on desktop PCs.
I've not tried it. Only got a 1.5GHz system here so would be wasting my time I think. Plus I don't have any games. Not sure if you can download versions of dubious legality anywhere, but I doubt you can just stick an XBox game in and hope it to work.