With such well-established guns as the CZ-75 and Sig P226, I don't really think there is a real "upgrade" between guns. Granted, the P226 has a better finish and more accuracy potential, but much of that is subjective upon the shooter.
I personally love my Sig P226, but I also regret selling my CZ-75. If possible, I reccomend just continuing to save until you can afford the P226 without trading in your CZ-75.