Notably phones are not really fast enough, or the subsystems aren't, to take full advantage of faster flash IMHO. I've used cards from class 2 through 6 and the only speed increase I saw was in the camera and loading your media player for the first time. And even when that was the case I saw no speed increase between a class 4 and class 6 card. Just something to take note in, class 4 seems pretty much instant in all regards.
If nothing changes on your SD as far as creating the libraries etc, you see no speed increase between class 2 or 6 when involving playing music or viewing pictures. You would in a faster machine however, but as fast as we think these phones are, they can't cope with much more information at one time that what a class 2 card throws at it. (I don't think this is a processor issue either, I believe the memory subsystem for the phone itself is rather sluggish, hence the reason the phone freaks out while installing a new app.)
Either way get what you can afford and if it works for you great! BTW the only time I've seen benefit in using a high class sdhc card was in a DSLR camera on continuous shooting at 30FPS. I don't think the Incredible will be doing that anytime soon lol.