They're all back in England, but I have many pretty valuable SF books, I collect them. The rarest is a 1985 hardback 1st / 1st of Ender's Game (only about 500 were in the first print run, and they mostly went to the book graves otherwise known as libraries), which would be $2,000 or so in mint condition but it's only a low VG so it's maybe $400-$500. I also have an NM signed copy of the first printing illustrated hardback of Terry Pratchett's Eric, which had a tiny print run for obscure reasons, a 1991 Ender's Game 1st (it had a revised edition in 1991...), an absolutely complete set of Peter F. Hamilton signed hardback 1sts (including the hideous US Mandel trilogy hardbacks, but The Reality Dysfunction is the rarest), the true first edition of Neuromancer (an Ace mass market paperback which is going up in value ridiculously lately, gotta be a bubble), some nice Iain (M.) Banks including Use of Weapons and The Bridge, a complete set of Lois Bujold...ahhh, books are shiny. I had my whole collection insured for about UK£3,000 when I was in college.
oh, yeah, and the two requisite stories of any book collector - the hard luck story and the good luck story. hard luck story first - I used to work in a small independent book shop in the UK, and I remember very clearly we got one of the (I think 300) hardback first editions of the first Harry Potter. When it came in I read the back and thought it looked kinda fun (I still sometimes read kid's books) but didn't buy it because I didn't quite have the money to spare. Last time I checked it was worth £10,000, I'm too depressed to check how much it goes for now. My good luck story isn't that great, but I found the first combined edition of The Lord Of The Rings in a second hand bookshop for £2 and sold it for more than £100 on eBay, that was nice.