I'm using a Gigabyte K8NF-9 motherboard that I'm quite happy with. Good support with NVidia graphics drivers for Linux from NVidia. If you decide to install a separate video card, I would recommend a Gigabyte with NVidia graphics, perhaps a 6600, because the NVidia site provides a well supported Linux driver for their cards.
My distribution is Mandriva, I started with ver 10.1 and I'm now on Mandriva 2006.
Do some reading about x86_64 before you start, there are some problems getting some 32 bit applications to install and run on a 64 bit system.
There does not appear to be a great speed advantage in running 64 bit versus 32 bit, some would say there is no advantage, but the AMD 64 bit CPU is very fast in its own right runing 32 bit applications.