This depends entirely on how you're measuring the systems against one another. The i5 2500k and 2x5850 would be the better all round choice, but for some heavily multithreaded CPU intensive tasks the 920 would best the 2500k. The 5850s are faster than the 4890s however you look at it so that part is a no brainer. The 2500k's platform is newer though so replacement motherboards are more easily available and less expensive, and they're more likely to include features such as USB3 if you're concerned with that.
