I am considering upgrading to 64 bit and have been recommended an Asus M2N-MX motherboard with an Athlon 64 3500+ Processor.
The supplier, however, in whom I have confidence, has admitted to no experience of Linux.

Is there a reasonably experienced user out there who has used this motherboard with a Linux system and can confirm that all elements function with the latest linux kernel and modules? Ie. video, sound card ethernet etc. I propose installing a gentoo system on it.

There are several motherboards that works in Linux. I prefer Abit or Gigabyte. Stay away from motherboards that have Marvell NIC and SATA, JMicron SATA and IDE, Silicon Image SATA, and Promise SATA and IDE.

People always recommend Asus M2N-MX from time to time. There are a lot better motherboards than this one.

I suggest the following motherboards.

GIGABYTE GA-M55plus-S3G
GIGABYTE GA-M61P-S3
GIGABYTE GA-M61VME-S2
GIGABYTE GA-M61SME-S2
ABIT NF-M2 nView

I like Asus and have had good luck with them. I guess I would worry more about chipsets than motherboards manufactures (at least about compatibility). A lot of Sis and Via chipsets have had a lot of problems. Nvidia seems (seems) to be about the most linux compatible. With Intel chipsets you have to be really careful, some are just great and others are a nightmare. I would suggest looking at the reviews at newegg. Even if you do not buy from there (pretty good prices) the reviews will give you a starting point on the worth of any given product.