If you have a spare disk then could carbon copy clone the existing boot disk to that and then use that as the boot disk.
Could then install a clean 10.6 to your original boot disk but not boot off it and then patch with the combo update to 10.6.5 that should have the drivers in it that you need. Then tell the Mac Pro to boot off that disk instead.
Or you could just build with the 3870 inside, patch the system and then insert the 5870 afterwards, whichever you think is easier.