As I pointed earlier that system should able to run 2gb of DDR memory without any problem being the maximum for the two dimm slots which is more then adequate for Windows there. http://support.dell.com/support/edocs/systems/dim3000/en/SM/specs.htm#wp1075776
As far as just what Crucial memory is compatible with the Dimension 3000 1gb dimms of either PC2700 or PC3200 memory run $30.99 at http://www.crucial.com/store/listparts.aspx?model=Dimension 3000
given the fact that he's seems to be a novice when it comes to computers, I don't think it's a good idea for him to start messing with his registry before finding out whether the problem is a hardware issue (not enough memory) or a configuration issue (registry, etc.), I suspect that it's probably because 256MB of RAM is pretty much insufficient to run XP...
I also have no idea why you linked the memory from the Crucial site, it's slower and more expensive than the memory I linked from Newegg...
First those two are simple automatic cleaners including options for restoring the registry. Those are generally safe and easy to use.
Not all memory will be compatible with a certain model prebiuld. You'll note the "guranteed compatible" statement there since most older prebuilds use value ram not any gaming/performance memory. On some older models you will find that you had to use single sided not dual sided memory seeing ic chips on one side only.
what's simple to you and me could be very confusing to a novice, people look at me like 'm crazy when I try to explain how easy it is to build a computer...
I've never run into any problems with memory, maybe I'm just lucky, also, I have a few friends who have the exact same model Dell and every stick of RAM I've installed to upgrade their system has worked, I just think those "guarantees" are a way to squeeze more $$ out of people who just don't know any better...