That board works with the Asus 7750 GPU, and the processor?
The board supports 125W TDP, so yes.
The GPU should, considering any PCIe x16 card will work.
And I'd reactivate my current Windows? I wouldn't have to re-buy it, would I?
Reinstall
and reactivate (you could test your current installation before this, your call.) You wouldn't have to purchase it again so long as you activate over the phone.