Amazon has some really, really great prices on microSD cards right now. In particular, the 8 and 16GB cards from Sandisk seem like a steal at $10 and $30, respectfully. It's almost a no-brainer to upgrade that 2GB card Verizon is giving out with the Inc at those prices.
Only time you MIGHT notice a difference between the Class 2's and 6's is when copying data from your computer to the disk (and only if you were copying a ton of data at one time.)
So you may see a tiny bit of improvement the first time you move your mp3 collection onto the card, but otherwise the difference (for this type of use) is negligible and certainly not worth the cash.
Think about what you are doing with the files on that card: playing songs, watching movies, opening documents etc. None of these activities is going to be noticeably impacted by the difference in read/write times from a class 2 and a class 6 card.
If i am running a 2 gig card on my rooted phone with custom ROM and themes, what do I have to do to change to a different card? Can i just copy all my files over to new card and put it in the phone? Will this affect my root? recovery img? ROM?