The quick answer, I suppose, would depend on your future plans for hardware. To begin with, in order to take advantage of more than 3GB RAM you will need to use Windows 7. Then, that depends if your Gateway machine is expandable. If not, then you might as well either stick with Vista or go with XP. That's either XP or XP Pro, by the way, as unless you're running a business server requiring advanced networking utilities, you won't notice any difference.
If I remember rightly, Vista was the one that had all the driver support problems (at least until they released the patches). Other than that, I don't have any direct experience with Vista - and neither do I wish to. I prefer to stick with XP.
Windows 7 is rated as the most stable of the lot so far, though might be using a touch more in the way of resources to load. What you'll find is that each release offers a good measure of backwards compatibility.
Personally - and given the specs of your machine, I would go with Win XP Home. But that's just me. When I have a question like this, I like to get a number of opinions to weigh up before making a choice.
Hope this helps.
Regards
Dr. V