If it’s old stock that he still has, it could very possibly be a legit deal. That is a very good price, since if you’re using say, SX3, it would cost you $499 to upgrade, anyway. I see this is Alto Music, and have 12k+ positive feedbacks, so I doubt it’s a scam. I would jump on it if you’re really serious.
You do realize, however, that you’ll need to go to Windows 7, right? Once you’re running Win7, you’re really going to want (as in need) more RAM than what your mobos are holding.