Gotta go with Vista here.
Windows Server 2003 x64 Edition has the same core as XP x64, so you'll have the same trouble. (That said, SP2 fixes a lot of stability issues...)
Windows Server 2008 I'd figure would be about the same as Vista but more costly. Although, features like Hyper-V look nice if you have the need for them.
Solaris x64? Odd choice, but you can't run WinUAE on Solaris.
