If you don't have a lot of time to learn about prospects, I recommend looking over Baseball America's Top 100 Prospect List . If you see a highly rated prospect who's baseball cards are relatively affordable, consider buying rookies of that player. Because their values are already sky high and practically have nowhere to go but down in value. I think Patterson, Hamilton, etc will be tremendous players, and their cards are decent investments, but you're running a huge risk if they don't pan out. If you stick to the Roy Oswalts of the world you run a low risk and a high reward.
Pitchers are notorious for getting injured .
They get hurt more than hitters do. In addition, even if a pitcher does well, his cards don't necessarily rise in value. To collectors, pitchers aren't as exciting as hitters. A player who can bash 35 homers is worth more than a pitcher who can post a 3.50 ERA, even though a 3.50 ERA is tougher to accomplish these days. I tend to shy away from pitchers like Ben Sheets, Ryan Anderson, Josh Beckett, Rick Ankiel, C.C. Sabathia, etc.