I think you're confusing resources and memory, and .jpg wallpaper with Active Desktop.
Active Desktop is much more stable and system resources (not memory) are unlimited in XP. A .jpg background will typically use less memory than a .bmp background. The catch is to make sure Active Desktop itself is disabled. This way you are using .jpg wallpaper without using Active Desktop. This can be accomplished through a varity of means, Group Policies is one that comes to mind.
As far as system resources and memory goes, the two are not related and the amount of memory you have in your system does nothing to increase/decrease your resources. For further clarifcation see the following FAQ:
I added more RAM, why don't I have more system resources?