I had Vista 64 with NinjaTrader and Esignal. It worked (mostly) but a few ninja bugs came up and 64 bit isn't officially supported at the moment by Ninja. Not sure if you'd have the same issues with 64 XP or not, but Ninja doesn't recommend 64 bit at the moment. I went back to 32 XP pro.
I used to use e-signal with Windows XP pro 64 bit version. The OS worked just fine with e-signal and Ninjatrader.
I planned to use the 64 bit version to run my trading computer only to discover that my brand new motherboard didn't support it. You may run into problems with sound which also means running an audigy pci card that 64 bit compatible. Also matrox support 64 bit but a lot of other graphics cards don't. It's can be a serious pain to sort out all these issues so do so at your own risk. Ask yourself if you're going to use more than 3.2 gigs of ram. That's a lot. I never use more than 1.5 running esignal and my brokerage software. If you want to access more ram, you'd probably be better to use vista and get something like vista lite to turn of all the unnessesary functions. Runningbear