"Would I be right in thinking it's just a matter of getting into the BIOS and getting the PC to boot from the DVD drive with the Windows 7 disk in then installing from there on?" - yes, that's right.
"Or am I being very naive and it's going to be a nightmare to install!" - should be fairly straightforward. Any problems, just post.
If thinking of going down the Win7 route, don't miss the Win7 pre-order deadline of 15/08/09. G