Thank you a lot. I have checked and the motherboard appears to have the latest BIOS, at least according to the relevant list on that ISO. There is a beta BIOS there too, but that appears only to enable hot-swapping (of sata disks, probably) which i'm not interested in. I just wanted to see if there was a way to enable a .5 multiplier.
Kind of sad to have weird multiplier inconsistency, but i'd rather have that than an incompatible motherboard or a motherboard i have to remember to tweak some settings every cmos reset or set some flags every kernel install or update just to have the darn thing boot (thing that have plagued my old experiences with Linux and 775/AM2 boards).