With a pawn rush, should I take the knight? I was just playing the computer, and it always starts with the knight on b1, moves out his other knight, and then plays his pawn. At this point, should I take his knight with my pawn, or do something else?

[fen "rnbqkbnr/ppp1pppp/8/8/3pP3/2N2N2/PPPP1PPP/R1BQKB1R b KQkq - 0 1"]
Take the c3 knight. Since white has developed only one piece, he can't punish you.  If he takes back with the d pawn, trade queens. Now he can't castle.  Then you develop, move your rooks to the center, and push him off the back of the board.