I went over a little bit, but you could throw a second 5770 in the build if you ever decided to play more advanced games. Or if you're absolutely sure you won't ever need/want to, you could switch the following:
can you play games on ubuntu? i know its free.. i would like to save the $100, and maybe upgrade later
With wine you can run basic 3D games like WOW and some OpenGL games, there should also be a beta version of steam available. But are you sure you don't have an extra/old copy of XP to throw in just for now until you get Win 7?
I'd go with the 450W unit. We're not talking a difference of 200-300W here, it's just 50W. You'd save a few bucks now with the Antec, but you'd lose money over time on efficiency and wouldn't have much room for overclocking or adding parts down the road.
That ASRock board is a good substitute. I'd still want the Gigabyte if it was available.