Hi
You could have a look at x-cart's own product configurator - that is in theory the mod for the job.
http://www.x-cart.com/product_configurator.html
You can see it even has a demo for a computer build.
However, don't get too excited - most people who buy this abandon it pretty quickly - you will probably need to modify it to make it actually useful.
For example (last time I looked) if you create a slot to add, say, memory, and you want 2 or more pieces, then you can't! Not really much use in the real world.