I just bought a new mb, cpu, and memory when my old 939 motherboard died. I can't get windows to install (it hangs at Setup is Starting Windows), so I'm trying to update the bios. Biostar does not seem to have a bios flasher that doesn't work without windows. I have a floppy drive, but no floppy disks.
I have the following setup:
Biostar TA790GX XE (bios date 1/13/09)
Athlon II x2 250
Seagate Barracuda 640gb SATA 3.0gb/s
Patriot Viper PC2-8500 1066mhz DDR2 4gb (2x2gb)
Diamond Radeon 4670 1gb
I have the same rig as you, and faces the same problem. I believe you could put the new bios in the usb and right after you hit power bottom, you would press F12 until its built-in flasher comes up (from mannual). Unfortunately, I messed up my bios reflash by using third party flasher (didn't know it has a built-in flasher until later) and have to RMA my board. Hopefully the new board will come within a week.