Yes, it will work. Why does DELL sell you the GX2 card that doesn't work with their system?
The XPS630 has x16 slots, but the actual bandwidth is only x8. So theoretically your card will work but its performance will be reduced in half. In reality, your card don't even utilize all of the x8 bandwidth (my speculation) so you may see no difference, or a slightly performance reduced, at best.