I actually did something like this with my IntelMac, as the "old" HDD had XPpro and a tonne of stuff I didn't want to lose at the time.
Believe it or not, most of the info was able to be imported to the "new" HDD, especially pictures, mp3 files etc. Documents were iffy, but most came over seamlessly.
Go ahead and hook everything up. There's nothing to lose, and potentially this is an easy way to transfer your unloseables to your new machine.
Either that or back up everything to DVD, but that will take a lot of DVDs, depending on the size of the old HDD and the info on it.