Hello,
The 18F452 has a clock quadrupaling (is that spelt right??!!) feature on it, this is rated up to 10Mhz (int clock 40Mhz), so you may be able to hammer this some more, I remember talking to a guy a PIC seminar about 5years ago that was running PIC's at 50Mhz (the 4wire powered can varity of xtal, like you seen on PC Mobo's) he said he had to run the PIC at 7V to make em work :wink:
The 452 has no usb uart tho, let me know please if you/anyone has any usb code for a standard PIC I could do with some direction....