4.Nxd4 is the best move, upon which Black has a few replies: Bc5, Nf6, even Qh4 is seen occasionally. White's main plan is to develop his pieces quickly and have good control of the centre.
EDIT: I forgot Bc4, the Scotch Gambit, which is also good with the aim of quick development again.
Try Nxd4, if Qxd4 you lose your queen. I think it goes 4. Nxd4 Nxd4 5. Qxd4 c5
Oops, sorry
4.Nxd4 is by far the main line while 4.Bc4 almost always transposes to a variation of the Italian game where White has played d4 quite early.
4.Nxd4 Nxd4? 5.Qxd4 leads to a nice White advantage as the Queen is not easily challenged from its strong central post. One continuation that has happened several times at scholastic level here is 5...Nf6 6.e5 Nh5?? 7.g4 +-.
I have seen people play this before, and I have tried it once or twice.
Any varitons of this, if so please list.