Outside of Ubuntu (and it's derivatives) Arch seems to have the best Steam and Nvidia support of any distro I've tried.
I got Arch all set up a couple of weeks ago, and then psyched myself out worrying about how stable it would be long term. After doing more research and talking to some folks, I feel those worries were unfounded.
I reloaded Arch, with Xfce4 and it's a pretty snappy setup. Nvidia works as expected, and so does Steam.
Why another switch? Kubuntu 13.04 was dog slow. It wasn't a throughput issue - I got decent framerates in games, and if I did a timed kernel build, it was in line with where it should be, but the entire UI was lagging. An example would be that Dolphin (the KDE file manager) took on average 2 seconds to launch. Even if I closed it and relaunched it right away. Sure, 2 seconds isn't a long time, but when you have a overclocked i7 and an SSD, you shouldn't have to wait 2 seconds for just your file manager to pop open.
Anyway, I'm gonna ride this Arch train for a while and see how comfortable it is. If I don't like it long-term, there is always Tanglu coming.