Hi there,
Juju: Gigabyte X38 DS5 should be Ok with Mugen. The Cooling is on a better side recently, not too big. I'd rate it with 95% compatibility chance.
cashman5: Might be different if you turn the Mugen CPU Cooler in a different direction. You can also mount the fan higher. It is not a big deal and I would not really rate it as "semi-compatible"
There are few ways to avoid the contact between the RAM and Mugen.
PeterStoba: GA-P35-DS3 should be fine, I agree. ASUS P5KPL-VM seems to have a simple design, should be fine as well. I will set both as compatible based on your supplied confirmation. Thanks!
Cheers,
Shinigami