DS3P has FireWire, it allows you to add a second video card (even though it won't work at its best, because only one PCI-E slot is running at x16), and it has only 2 PCI slots.

If you don't care about FireWire or a second card get the GA-P35-DS3R. It has 3 PCI slots, btw.

DS3L is a good board too, but to make it cheaper they cut some features. No FireWire, no second video card, no RAID, only 4 SATA devices instead of 8. If you can afford $30 more get the DS3R.