the domains by themselves i don't think have any real value in them.
of course developed, you will have to work how much time you put into the sites, the amount of content available, traffic etc to come up with a price to sell for :/
even if developed, i don't think the domains would be worth much unless you built up the domain brand.
most of the people when buying developed domains/sites would pay for the brand, traffic, content, revenue earned, etc.. not neccessarily the domain name.
i dealt you will get much for the domains unless you have something somebody wants on them.(but i don't know what you would use these domains for), so cant comment on what to develop on them.
With kustompc.net you could play off the idea that you like to be 'different' or devious (because of the K in the spelling) and perhaps aim the site towards gamers who like to customize their PC cases or something.
That sort of audience likes to feel like a rebel and quite frankly likes to spell things wrong - so they would probably love the name and relate to it.
I would say that this name is the best of the two you mentioned... develop it towards a custom PC case resource or something like that and it could certainly be worth something... to someone out there.
But by the time you've done all this developing why not just keep the site yourself and put some ads up – you should, by then already have a good audience aimed towards PC Gamers, Techies and PC hardware freaks etc... so you might want to keep the site and see where it all goes.