That's a very similar setup to what I have here. I have the A7N8X Deluxe Rev. 2 board, athlon xp 2600+, and an MSI fx 5600 card.
For Mandrake, I've found that you have to pass the kernel the options 'nolapic noapic' to keep the machine stable, or you can just disable apic in BIOS. Disabling the BIOS option seems to work better. Without disabling APIC in either BIOS or kernel parameters, the system will lock up during at all heavy disk activity. Install seems to work ok without disabling anything though, and other distros work without disabling apic (Debian and Fedora), so I think it may be some patch Mandrake uses (or doesn't use).
The onboard sound is (supposedly) supported out of the box via ALSA. However, I haven't tried anything that uses more than 4 speakers of the 5.1 I have, and there is a LOT of static for some reason. I'm not sure where to look for fixes on that. If you find any solutions, please post!
There is a 3com-based nic, and some other one... the 3com nic works out of the box with 9.2 for me, the other will probably work with the NVIDIA drivers, but I only have a use for the one at the moment.
If I could get the sound issue dealt with, I'd be elated...this setup is very nice. If the SBLive works properly, I could just go and get one to use rather than the onboard sound, but it works so well under windows, I'd rather be able to take advantage of it under an OS that I enjoy using :-)
Good luck with the new system!