Topalov was very unlucky to lose the first game with Black. In the opening he was slightly worse, and it looked as Kramnik had reached one of his trademark squeezing positions, but Topalov managed to sacrifice a pawn for very active counter play. He had a sure draw by a repitition of moves, but found some amazing resources to try to win - only to blunder and loose.
In earlier years Topalov was very upset after such a game, but my impression is that he is now much more able to get some positive energy out of his emotions. As he said in an interview in NIC magazine some time ago, he is not afraid to loose. And we have not forgotten his amazing come-backs in the tournaments in Sofia and Linares. So we should see another interesting game two.
What opening will Topalov play against Kramnik?
Topalov, like Kramnik, can play both 1.e4 and 1.d4.
Against 1.e4 Kramnik plays the Berlin or the Petroff, and if he wants some better chances to win with Black he plays the Sveshnikov. In a match, and especially with a lead, I would expect Kramnik to play the Berlin or the Petroff. Kramnik has been extremely solid with these openings, especially in matches. He unnerved Kasparov with the Berlin in their match. Kasparov never got anything from this opening, and he switched to 1.d4 very late in the match. Against Leko he played the Petroff, and Leko very quickly switched to 1.d4, although Leko never really had been a 1.d4 player before in his life.
Is this so scary that Topalov will play 1.d4 throughout the match? I think Topalov will consider his knowledge of both 1.e4 and 1.d4 with White a valuable asset, and will have prepared something against the Berlin and the Petroff, too. Kramnik may actually prefer the Petroff against Topalov, because Topalov plays the Berlin himself sometimes.
Which line could we see in a Petroff? After 1.e4 e5 2.Nf3 Nf6 3.Nxe5 d6 4.Nf3 (Topalov even tried 4.Nxf7 once against Kramnik, but I am sure we won't see this ever again) Nxe4 White usually plays 5.d4, although in recent years 5.Nc3 has also become popular. After 5..Nxc3 6.dxc3 we often see positions with opposite castling, which lead to less symetric positions, and to much more dynamic attacking chess that Topalov may prefer. While practical results with this system have been good, the situation is much less clear theoretically, not the least because White's center has gone, which together with the open e-file should Black give good chances, too.
In the main line Petroff, after 5.d4 Kramnik usually plays the rock-solid system with 5..d5 6.Bd3 Nc6 7.O-O Be7. If Topalov goes into this line against Kramnik, he would better be prepared extremely well, or he will just waste one game with White.
Against 1.d4 Kramnik plays the Nimzo Indian. If White plays 1.d4 Nf6 2.c4 e6 3.Nf3 Kramnik usually plays the Queen's Indian, although he sometimes goes for the classical Queen's gambit. Topalov has had some fantastic games with White against the Queen's Indian in recent years, where he brought with his attacking ideas new life to an opening which was previously considered very drawish.
So what do I think will they play in the second game?
Unless Topalov has run out of ideas in the Queen's Indian, or found something impressive against the Petroff, I think we will see a Queen's Indian.