Most PC sounds are only stereo. The old soundstorm control panel used to have a check box to create the center by mixing the two front channels. If you still have the old control panel on system, that check box will still work. Check in the Nvidia folder. If you dont, there is a registry...
This is a known problem with the Audigy. Check out this long thread in the Europe Creative forums.
http://uk.europe.creative.com/support/forums/thread.asp?thre=13916&foru=51&page=1
Both of those motherboards have Soundstorm, which means they can encode anything with 5.1 channels into Dolby Digital. This means you will get surround sound in games by connecting the SPDIF on the motherboard (optical on the Abit, coax on the Asus) to your receiver.
I have the NF7-S...