My biggest problem is space. I have a wife and two toddlers, and my computer desk is in the space where all the kids toys are, which has turned into a heap so my desk is also over-run with odds and ends. I really need some shelving above my desk for all the miscellaneous junk.
But as far as usage of the systems I want to build, I'm really only interested in a handful of games: Chaos Overlords, Tie Fighter, Missionforce: Cyberstorm, and some other typical games which all easily find themselves at home on Windows 95, IIRC. So it shouldn't be much of a task to build an appropriate system, but I also can't neglect family (much) in favor of system building.
I'm trying to liquidate my stuff, for the most part, right now as all my stash of parts and systems are at my parents' house, and the owner just sold the house so they gotta move, soon. Ah! So much stuff. At least making the decision to get rid of all the P4's is easy. I'm only keeping ones which are heavily loaded with features for use as Linux machines, but even those I'm doing my best to let go of.