Having recently 'upgraded' from a 2.8ghz p4 on XP Pro to a Q6600 on Vista my comment would be "Don't expect a quantum leap in performance". Yes, it is better running multiple apps but I think that Vista kills a lot of the potential of the system. i have turned off all the 'bells & whistles' so to look at my system it does not seem really different to XP. I am probably going to get another hard drive and put a clean install of XP on it if I can locate all the necessary drivers. I will then be able to really compare the difference a bit more accurately. At the moment I would say that you would probably notice as much difference if you increased the memory on your current machine.
If you're planing on using Vista 64bit as opposed to 32bit then more memory might give better performance, If you're going to use 32bit then 3GB is about right. A 32bit system can only access 4GB total ram, this includes graphics ram, so even if you have 4GB installed it won't all be used. I myself have 4GB installed on my Vista 32bit machine with a 1GB GeForce 9800 X2 graphics card and the CPU Usage Sidebar gadget reports 3070MB Total ram fitted click here, with the ram on the graphics card taking the place of 1GB of system ram.
Fitting the additional 1GB may not give the performance increase you may expect.