I have no idea what the best MB for 3400+ is. I went with the EPoX 8KDA3J, and I think I'll buy more of them. This is a no frills, basic nForce3-250Gb MB that has most of the essentials. SATA, RAID, Gb-LAN, cool 'n quiet. If you want/need more features, there's a more expensive '+' version, 8KDA3+, I think.
Only bad I have to say about it, is that it was difficult to install 3'rd part heatsinks (Stock cooler installs like a breeze). But it was entirely due to the makeshift solutions for socket 754/939, by the heatsink manufacturers. And I'll guess the problems are the same, whatever MB you choose. In the end, I was greatly helped by the backplate&frame coming with the MB. With other bolts and spacers, I was able to use them instead.
I also think the newer fibreglass backplate for Zalman 7000 makes it easier (the metal/paper one is useless). I've ordered such newer cooler for the next, and will know soon enough.