XP64 is the fastest, hands down. XP32 is second. Those claiming Win7 is as fast have drank the kool aid, it is just Vista SP2, and not really that fast at all. I mean, it's better than Vista, but so is putting my own fist in my own ass.
XP is still king by a very large margin!
Uh, Windows 7 is just as fast as XP if not faster. There are several benchmarks as well as feedback from several members here and from other forums to confirm this in addition to the benchmarks.
Just because it looks similar to Vista, does not mean it should be referred to Vista SP2 and regarded as such. Windows 7 and Vista are two completely different beasts under the hood.
Windows XP was technically Windows 2000 SP5, and Vista was technically Windows XP SP4 going by that logic.
There are several improvements both compatibility and performance wise over Vista that should blow XP clear out of the water. Even in the beta and RC stages it is still much faster, better and stable than Vista currently is and will likely ever be.
Even those that bought into the Vista fud or were originally against Vista, are now saying how much better Windows 7 is than Vista.
Also, Windows 7 runs quite well on older hardware without any issues, the same thing can't be said for Vista. Of course you won't get the flashy Aero interface if the video hardware doesn't support it, but the operating system itself will run quite smoothly on older hardware
The features, increased performance, compatibility, and stability over Vista in Windows 7 are a good reason to finally ditch XP.
There is no real reason to stick with XP besides being completely ignorant or needing XP for the odd game or few which won't run on the newer operating systems, in that case dual booting should be considered.
In some cases you'll find that odd game, application or few will likely run in a virtual machine. Windows 7 will have a Virtual XP mode that will be available as an update to keep the compatibility gap between XP and Vista/7 relatively small.
Sure Windows 7 won't shine much in emulation, emulation is already so resource intensive that whatever performance benefits an operating system offers, it is unlikely it would make much of a difference in emulation.
However PC gaming and general operating system usage is where the benefits over Vista and possibly XP will be noticed.
Microsoft has even announced that they are continuing to optimize Windows 7 and fixing most/all the major issues (Compatibility issues, Stability issues, Performance issues, Bugs) that everyone had with Vista to ensure a successful launch.
Windows 7 performance is already great compared to Vista, performance is already comparable to XP or better and will likely get even better before the official release of Windows 7.
In relation to what 64-bit operating system is best, stick with XP64 for now until Windows 7.
There is however no much of a difference in performance with either Dolphin or PCSX2 regardless of what operating system you use. I generally get the same speed in these emulators no matter if using XP, XP-64, Vista, Vista-64, Win7, Win7-64.
Also the performance benefits the x64 build of Dolphin offers over the x86 build is relatively small, about an 8 - 15 FPS increase with some games.
The x86 JITIL build can run most games with performance and speed comparable to the 64-bit builds of Dolphin.