Object Desktop can take up variable memory depending on which OD apps you have running, and which skins or themes you're loading with each.
For example, most WindowBlinds skins take up very little RAM; but there are a few, depending on the graphics used and some internal skin settings, that can take up to as much as 8 or 9 MB (though that is rare). Most use less than what the system would assign to Windows by default for drawing and maintaining the normal Windows XP "skins".
Add in DesktopX and things get very variable. Some DX themes take up as little as 400Kb of RAM, while the one I run (custom) takes up roughly 18Mb. Just depends on how many and what kind of objects, animations, and plugins you're letting run constantly.
Same with ObjectBar. WinFX, another OD app, can take up a few Megs if shadows and transitions are enabled.
IconPackager seems to be the exception, at least as far as I can tell. It only seems to eat RAM when actually changing icons and loading packages (which it only does when you tell it to). Otherwise, it doesn't take up any (or at least not much) RAM during the course of a Windows session.
The good news is that you're running Windows XP, which doesn't have the nasty ol' "System Resources" and "GDI Resources" problems of the 9x/ME line. XP dynamically allocates RAM to whatever apps need it the most (it prioritizes, but doesn't have "reserved" memory, a throwback to DOS).
With 256MB, you might see a slowdown while running several OD apps on top of using Maya or other intensive graphics apps. Adding another 512 would probably eliminate any slowdowns, though, except when rendering or compiling large graphics or movies (which will slow the system down whether or not OD apps were running...

).