His first card, chronologically, is in the 2005 BBM Rookie Edition set (#12). I think it's safe to say that 99.99% of current rookies get their first card in this annual issue. After that, he appeared as part of a multi-player card featuring the team's draft picks in the "Challengers" box set (#30). And he was included in BBM's 1st and 2nd Version sets (their flagship release) on card #116, with silver and gold signature parallels, and #760.
Other rookie-year sets include two cards in the premium Touch The Game set (#25 and #147), with his first relic card - a game-used ball card with a logo parallel, and a photo card. He also has three Konami game cards.
There's one more rookie-year set that Darvish appears in; he has two cards in the annual Fighters team set. Card #F03 is on the left, below, and subset card #F72 is on the right. The team set, issued in packs, also includes Darvish's first autographed card, a parallel of the base set limited to only 10 copies.
His rookie cards are definitely the key cards in every set above, and while they aren't as expensive as one might pay for Pujols or Ichiro RCs in the US, they usually don't come cheap. But thanks to a poorly-listed auction on Yahoo Japan Auctions, I got both of the above cards and many more singles from Japanese players in the MLB for under $2 shipped. In fact, I didn't realize I was getting Darvish's rookie card until tonight, when I went through and pulled out the singles I needed from the lot.
Even without the RC, I certainly got my money's worth, as several cards went to a small collection I'm working on, and a few others went to my type collection - including that Newcomer parallel above.