I use an alternative to Windows, Tiger! Soon to use Leopard. Very few limits with software.
A friend of mine is trying to use linux for much the same reason you are, and is finding issues with software, especially photo related.
While he can use gimp, there's a lot of features it doesn't have which he'd like and there's just no alternative.
His last project was writing his own HDR software to run under linux after completing what he thought was a more efficient blur tool.
I think in the end, depending on what you're doing, you're going to get the most functionality out of a Windows or Mac machine, especially if you're wanting to run photo editing software.