You can do it. In fact i've done just that recently. But, it may not always work to well... so lets just say you've been warned.
Also there's several things to consider before you do it:
1. The old drive may be incompatible with the new motherboard. Not likely, but possible.
2. The old drive is just that - old. With age comes potential reliability issues compared to the new drive. Also, the old drive almost certainly will perform less well than the new drive and have a noticeable effect on your new conroe system.
I would recommend using something like ghost or perhaps partition magic to clone your old hard drive image onto the new drive, so you get your old OS and the benefits of a modern/new hard drive. You really need to know what your doing to do this properly.
When you have swapped or cloned the drive and are ready to boot the old OS, make sure you boot in safe mode. Then uninstall all the old Motherboard drivers and integrated peripheral drivers etc before you do anything else. Reboot in normal mode. Install the new motherboard and new integrated peripheral drivers. Then, in theory, it all should work. But i'll give you no guarantees... Ive tried this a couple of times before, once it stuffed up royally and kept hard resetting randomly, the other time, it worked fine... All bets are off.
Personally, unless you have a very good reason to do it, i would do a fresh install. Seriously.