I'm leaning towards the mobo+proc at the moment, simply because I'm afraid the GA-MA790X-UD4P will start selling out, being a ddr2 mobo and all. Also, it seems logical that when nvidia releases their new cards, that it'll drive down the price of the 5xxx series a bit.
However, the thought of getting a 5850 is still very appealing. How can I really tell if my CPU is a bottleneck? Is it *only* when it runs at 100% load (or 50% for a single core)?