They would have a lot to lose by ripping off customers, vs. a random seller on silkroad, so I'd feel comfortable up to a few thousand dollars.
I think I've actually bought something from Memorydealers before. I'm not sure of the relationship between bitcoinstore and Memorydealers. If they're the same entity, then I'd be fine up to $100k or so -- it's a real business.
My credit card issuer effectively acts as an escrow agent, and I love that fact when ordering from companies. It has saved me more than once, too.