eMacs come in 700mhz up to 1.42ghz varieties...
If you're going to need to run XP, you need the fastest machine you can get your hands on, performance is barely acceptable on a PowerBook G4 800mhz. However, if you are able to compromise on the Windows operating system, and run Windows 2000 (or Windows 98SE) VirtualPC is pretty usable.
You will need 10 gig of disk space available to be able run VirtualPC for any length of time - the installation seems to bloat over a period of weeks usage.
Though VPC is comparatively slow in operation, it does have a neat time saving trick - saved states - after you have booted Windows in VPC, you have used the program, and now wish to shut down, VPC can save the state of Windows and any running programs when you quit VPC. Next time you start VPC, the prog will boot back to the place you left Windows very quickly.
Personally I used to use VPC 7 running Windows 2000 on my PowerBook G4 800mhz - it's only purpose was to run Garmin Mapsource....
If cost is an issue, a cheap pc from ebay or a computer fair may prove cheaper, but, will also take up a whole loada space too...
Just a few thoughts
-Rob