It probably will come with 3.1.3, but since it's a 2G, it should be downgradable. To do so, you have to download the old firmware, and point itunes to it. If you downgrade it to 3.1.2 it will be jailbreakable, but it will be a tethered jailbreak (you have to hook it up to a computer to reboot)
Honestly, I'd recommend buying a 16GB 2G from the refurbished store. That'll have the old bootrom so you can always jailbreak at any firmware without being tethered, and you'll get twice the storage for the same price. With the apple refurbs, they change the back, the screen, and the battery so it's practically as good as new.
edit: seems they don't have the 16GB refurbished anymore. too bad. In that case I'd probably go for the 32GB since it's a lot more storage for only $50 more.