Regarding emulation... 733MHz isn't too bad. Linux can run natively. And if you plan to emulate something that requires 1.4GHz then I can almost guarantee that the measily 64MBs RAM will stop you first (e.g., Mortal Kombat in MAME).
Upping the processor speed to 1.4 entails a couple things: One, that is likely a P4 (probably a Willamete or an early 0.13um Northwood). Xbox wasn't created to work with the P4 socket or power supply. Xbox was designed with a Celeron P3 in mind. Even the faster P3's (Tualatin's 1.2's) use a slightly different socket. Second, is the Xbox chipset/bus compatible with P4? Possibly, but not guaranteed. The IO speed should be sufficient since xbox rarely saturates the IO. Not too mention there's not a whole lot of disk access during game-play other than load times.
--Phot0n
xbox 1.0 w/ Matrix & Evox
xbox 1.0 w/ Chameleon & Avalaunch