AM3 compatability?

AM3 processors are compatibale with Am2+ Motherboards, but sometimes require a bios update.

This ASUS site states M4N78-AM supports AM3 processors, and in particular Phenom II. It may be possible that the 955's TDW (max wattage) may be too high tho'.

http://www.asus.co.nz/products.asp [...] odelmenu=1

I think you nailed it brown63. I scanned the list and not a single 125W processor was listed, even the old ones. Back a year or two ago, it was brought to light that alot of these boards couldn't really support the high power CPUs, especially when OC'ed. So to answer your question, it might be able to do it, but it is somewhat risky and you would definitely want extra cooling over the power circuitry. If you're going to put a high spec processor like a 955 in, you might just want to cut your losses and get a new board. Or, if you don't already have the 955, get the 945 (which is supported).