It really depends on what sort of development you're doing. If you're doing something that requires massive amounts of memory & resources (huge datasets and lots of math), XP is the way to go. Even if you lose some of the memory by not running x64, you're still gaining about 750mb that would otherwise be chewed up by a lot of the Vista background services.
If you're doing web development, it depends on your testing environment. If you do everything locally using IIS & VS.NET, it doesn't really matter. VS has it's own webserver built in. If you have to test on IIS, it's handy to keep a virtual machine around to throw it onto a 2003 server. Keep in mind that Vista will eat up some of that memory that could be used for a virtual machine.
Given your list of requirements, I would have to recommend going with XP Pro. If you're a heavy multitasker, you need all of your resources in front of you, not being chewed up by the Aero and background searching. I used Vista for development for about 8 months, then I got a new job where I had XP. At first it was annoying, but the responsiveness of XP trumped the features of Vista, especially when there are tons of windows open and all my RAM is gone.
There is also a compatibility issue: a lot of VS.NET (especially older versions) are fairly... difficult under Vista. For instance, the publish feature for web sites can be broken under Vista. You also have to be careful about how to deal with the UAC, as some VS.NET stuff requires you to explicitly run as Administrator. I also recall having some difficulty with Microsoft Virtual Machine, but this was in 2007. I'm sure that's patched up by now.
