Another vote for the P5Q here too
As for new O/S, have a look on OCC (overclockersclub) and you'll find a sticky somewhere on how to swap hard drives without having to re-install, it basically involves removing the drivers for any differing hardware between the systems and as a parting shot removing the chipset drivers for the old mobo.
This will cause your system to very likely not boot.
Build next PC, add hard drive, boot up and install the chipset drivers (ICH10 in the case of the P5Q) from the disc. and you should be good to go.
However there's nothing like a proper install to clear the tubes, run Belarc Advisor to find your licence number and then borrow someone elses disc, Install Windows from that and put your number in when prompted, if it prompts you to do the automated phone call thing then do that..job done.
No laws broken, you've paid for the Windows you are using, just upgrading hardware that's all.
Edit, I see you've got your own OEM disc, just let it prompt you for the telephone registration, I've never heard of it being refused because of upgraded mobo's.