I don't know about CycloDS , but i've got a 8GB Sandisk (class 4) in my DSTT and it works flawless.
I even tried Both Castlevanias and both intro videos was lag-free, i played both games for about 1h smoothly ( except that PoR froze on me once.. but.. i guess i have to live with that)
Oh, and btw, i've formatted it as FAT32 64KB Clusters, but i don't know if Castlevania would've worked without that tweak.
So, The card itself is good.