FYI, I don't know about games but I tried a single 150 Raptor for OS X and applications. I tried it out for a week and found it to be not worth the money. I've just been issued an RMA and it's going back to Newegg tomorrow.
What I found while using the drive was that OS X loaded a second faster. It also shaved a few seconds here and there on the launch of applications (but the MacPro is already so fast it's hard to tell). The trade off was EXCESSIVE seek noise and high price per MB. The drive itself is quiet when idle, but when you do anything that requires disk access, it does a LOUD rata tat tat tat... like microwave popcorn. My MacPro WAS virtually silent until I put that crap drive in. I'm trading it in for a WD Caviar 320. The new drive will be a lot quieter, cheaper and have I will have more space.
Those of you who say the Raptor is not that loud.. I call BS. Uh.. compared to what? Your lawnmower? Well, maybe if you listen to loud music or work in a sweat shop then maybe. Noise sensitive buyers beware!
Finally, I think the application for the Raptor is limited. The only valued use I see for it is maybe two of them in an (expensive) raid for games or scratch. Any application that requires fast disk access. But for OS and general computing... it's overkill. Who wants to listen to tap tap tap when they are surfing the web? I'd even say the speed of the drive is not really that noticeable unless you have a slow machine. The MacPro is already wicked fast and the Raptor is NOT going to make it that much faster. I boot in like 5-10 seconds already! You are better off buying more ram or setting up a simple raid for scratch. That's what I'm going to do! And it only cost me $30 restocking fees to learn my lesson. I got suckered into buying the Raptor because of various posts that said "the Raptor is the best investment in my computer I've ever done" and other mis-information like that.