I've got the same motherboard and case and they're doing great together. As far as aftermarket heatsinks go, the Thermalright Ultima-90 just barely fits in the case if you want to do some serious overclocking.
I bought my Ultima 90 from Silicon Valley Compucycle. Great service and speedy shipping. :)
Ultra 90:
http://www.svc.com/ultra-90-k8.html
Ultima 90:
http://www.svc.com/ultima-90.html