The 64 bit CPUs are a little pointless today, because everything is still running in 32 bit. With that said, more and more drivers etc are being written for 64 bit every day, and the next major Windows release will run much better on it (if you get the 64 bit version), so it's a good way to do a little future proofing of your PC.
That said, it's hard to get a non-64 bit CPU these days If you have a little bit extra cash, a dual core (or "x2") AMD chip is money well spent for the future too, but even for current systems it's a good idea. Although applications need to be written to take advantage of the dual cores, your operating system can already, and if nothing else it means the OS and all your background tasks can run on one core, your game or whatever you're doing gets the whole other core to itself, meaning no overheads and true multitasking. Again in the future applications will be written to take proper advantage of dual core, and your PC will be so much faster for it.
As for HD and motherboard, my best recommendation is a Seagate drive (get a SATA motherboard, and a SATA drive, it's much faster than normal IDE). If you're buying a new motherboard, you might wish to consider new RAM also, as the new board will undoubtedly support much faster RAM than you have if you're looking at upgrading, and it really does make a world of difference.
I've had a LOT of trouble with Gigabyte motherboards, however ASUS, Abit and MSI come reasonably highly recommended. Go for one with a PCI-E(xpress) slot if you plan on buying a new graphics card to do any gaming.
And lastly, the 19" Samsung 913B LCD screen is an awesome monitor if you need a new one of them