I'd say that if you don't have any current problems with XP, don't switch to Vista. Let me first say that I have little personal experience with Vista. However, from the reviews I've read (professional and user), people either love it or hate it and there's little in between. I personally don't see a valid reason to switch to Vista at this time as there's nothing new that appeals to my needs. For instance, MS had originally planned an entirely new filesystem (WinFS) that was to be reportedly much faster and better than NTFS, and if it had been implemented then I would probably recommend it. But, as far as I can tell, the improvements are mainly visual and if you want a better GUI, you can already get third party apps to make your interface however you want it in XP.
As for dual-booting Vista and XP MCE, it's certainly possible. But, like Firehazard mentioned, there may be some errors if you try to run certain programs in Vista from your MCE drive. Like he said though, this will only be limited to programs that store/access data in the MCE registry. You could always dual-boot though and simply choose between Vista and MCE when you boot the PC. However, if you go this route, XP MCE may lock you out if you simply pop the HDD with MCE into your new box. If I recall correctly (never faced this issue) if enough/certain key hardware components change (mobo, CPU, RAM...), XP will lock down and not allow you to do anything because it figures you've gone and pirated it. I do remember reports about this when XP first came out, but this may have changed.
Anywhoo, if you like XP MCE and have no current problems with it, stay with it until Vista shapes up better. You can always upgrade later and it'll save you from possible headaches now.