the CPU and GPU are both excellent choices, but i would hesitate to recomend an asus MOBO. i have had bad experiences with them, and know of many others who have had the same.
id recommend going with a gigabyte or biostar board. the Mobo im using (in my sig) is excellent, though im not sure how much it costs in the US. certainly worth it if within your budget.
Nothing wrong with Asus boards if you ask me. They and Gigabyte have always seemed to be pretty reliable. Biostar, on the the other hand, I've heard a lot of bad things about.
I would absolutely not downgrade to a triple-core processor to save $80 if you're already investing in a 5850. Although you might be able to save some money by looking for a CPU/mobo combo deal.