Hmm, that's a tough one. If we're counting anything with Castlevania on it, I would say Issue 2 of Nintendo Power. As far as the first thing I bought specifically for collecting purposes, probably the CD soundtrack of the first three games (not pictured).
I know what you mean about space limitations. My wife and I just got rid of a couch in the basement to make way for some more stuff, same with a table that was down here. Everything is pretty crammed right now, lol. The finished part of the basement is easily the biggest room of the house, but the total square footage of the house itself is pretty small, about 900 sq ft if I remember correctly.