I have a small webserver running on a 400 Mhz Celeron, with 256Mb Ram and a 40gb hard drive. It is running FC2.

I have the chance to migrate most of the hardware into a new box, which is a 800Mhz P3. I can move over the RAM and the Hard drive.

Can that be done, without re-loading Linux? I don't want to migrate all the hardware, only to find out my machine will be locked up afterwards.

Any idea if it can be done?

I have not done this myself!!

I read about harddisks with a pre-installed OS, which means they are not very hardware specific. There are some (minor) issues you might run into e.g. if you change the video card, but that can also be done in the same box by simply installing the correct drivers. There's a lot in FC that autoconfigures and auto installs (like USB flashdisks and cameras, printers) so I don't think you'll run into any major problems. Still, this is only an educated guess. Why don't you simply plug the hardware into the new box and see what happens. You can always plug it back.