I was concerned that I would have hard times with Vista Ultimate, but I popped it on anyway. I only have 2 gigs of ram, that was my concern.
Vista gave me a huge performance boost in Age of Conan and Mass Effect. I was very impressed.
AoC usually chopped at about 15 - 20 FPS, it's not easily over 30 fps and all the settings are on high.
Mass Effect used to be blurry at a distance, now it's nice and sharp.
I want to note, that I do not have a DX10 video card either, I have a radeon x1950 512, so it's a big card but still isn't crossfired with another or on Dx10.
Vista launch tanked, heck it train wrecked. But SP1 seems to be treating me well, as I no longer run it on just my laptop but one of my home PCs as well. Just tweak all the security warnings out and make it look like XP and it's like a really nice XP Pro Box.