UPDATE:
Ok, I've tested both castlevania D.O.S. & P.O.R.....
Kingston (Class 4) 8GB MicroSDHC
P.O.R.: Seems fine.
D.O.S.: Got very slight slowdown right at the end of the title screen video. Plus; graphical glitch on first boss (* see note below)
Kingston 2GB MicroSD
P.O.R.: Seems fine
D.O.S.: Intro & game plays flawlessly
* note.
I got a few graphical glitches running a trimmed version of D.O.S. on both cards.
Using a clean version of the rom gave no glitches on the 2GB (MicroSD) BUT I still got a garbled image of the first boss using the clean rom on the 8GB (MicroSDHC)!