Actually, I don't have a genuine Apple store - I have several resellers, one of which sells exclusively Apple products. That's the one I'm planning on going to. I'll have to ring them and see if they'll transfer my library over. The rest of stuff I don't care about, but it'd be a shame to have to re-buy all my music.
I guess you could also consider asking to borrow an external hard drive or ethernet cable to do it yourself - if you're planning on spending a lot of money with them, it's not a lot to ask. Depending how you feel about it, you could also consider buying one and then returning it as "not working"
Otherwise, transferring across a wireless network should be nice and easy and could be left to run overnight. Never thought of this option (I own several large capacity external hard drives so I guess it's a case of when all you have is a hammer, everything looks like a nail!!)