I have this same board with iPC 10.5.6 Final running great. Even on real Macs OS X can be picky about ports so first I would try different ports as some of them might not be running at 2.0 specs. Generally the ports on the front are not 2.0 so try to use the back ports for high-speed stuff.

Also check your BIOS USB settings for stuff like OHCI and EHCI (1.1 and 2.0 respectively) and make sure the high-speed is enabled.

If that doesn't work install the modified SB700 kexts from this thread. In my case the AMD/ATI chipset drivers from the iPC install work just fine but it might help.

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iPC 10.5.6 w/ Voodoo Kernel | AMD Athlon X2 5000+ BE @ 3.01 GHz | Gigabyte GA-MA78GM-S2H | 780G North & SB700 South | ATI Radeon HD 3850 | 2GB DDR2 800 | 2 x 500GB SATA HDD | 1 x DVD-R DL SATA