Comics that are signed by a creator are usually worth no more and no less, unless you can find an individual who's a big fan.

Actually, the Neil Gaiman issue of Spawn is worth more than the Dave Sims issue, mostly because Gaiman introduced the character Angela in that issue. There's an interesting legal battle that later ensued (that was wrapped up just recently) that goes, yet again, to prove that Todd McFarlane is a big asshole.

You might want to go over to Comics Price Guide. They seem to have not-totally-unreasonable price listings available for free.

You definitely want to sort your comics. Generally, the less trouble you put a comic store through, the more you're going to get. Constantly flipping back and forth in the price guide is a big pain, and whoever's doing it will get pissed off.

You should probably also make a list of what you have and copy it for however many shops you're going to go to. (Call first, though.) My shop wanted to see the comics in person, but many others prefer to just get a list initially to determine if it's stuff they want. There shouldn't be a problem with shopping them around, but I wouldn't try to start a bidding war unless you've got an Incredible Hulk #181 or something.