The restore file in the download section is a legit 3.1.2 (or whatever version you downloaded) IPSW straight from Apple. Upgrading via that will un-jailbreak your phone (on an iPhone 2G you don't have to worry about a software unlock solution becoming unavailable - for now and probably forever).
I'd also recommend just staying with 3.0, since 3.1 doesn't offer that many improvements and you'll have to redo all your settings and things with Cydia.