My guess is that the only reason you're holding back on 8GB is the class right? To put it simply, there's less to it than meets the eye. Lemme explain...
I'm using a Transcend class 4 8GB, which is just a little slower than ADATA. I get no lags or freezes in games or intros, even the DoS castlevania intro runs smooth, although PoR still freezes on teleport stones occasionally. I am however using the DSTT menu. On the SC menu the DoS intro lags but again, nothing else changes - all other intros work fine and PoR still freezes occasionally, though not any more often than with the DSTT menu.
Back on Acekard 2 I used a class 6 Transcend 4GB microSDHC and it STILL lagged in the DoS intro, though it didn't lag at all on the class 6 ADATA 4GB, even though the Transcend was FAT16 and the ADATA was FAT32 so clearly the brand makes a huge difference. Class rating is all about sequential access, which is what movie intros need, yet even though both were class 6 the ADATA behaved better.
My guess is that if you get an ADATA class 4 8GB you have a 50%-50% chance of getting lag in the DoS intro, no lags or freezes anywhere else, except occasional freezes on PoR warp points. If you get the class 6 ADATA 4GB you're guaranteed not to have lag anywhere in any game or intro, and have a 50-50% chance of seeing occasional PoR freezes on teleports, which may or may not happen at all.
My recommendation is that you go for the 8GB, since aside from these two games you're not bound to have trouble anywhere else, plus the differences in performance on said two games are neglectible(unless you REALLY love the DoS intro), just save before using warpstones in PoR. Also the DoS intro lag can be forfeit altogether if you're dual/triple booting the SC OS with YSmenu and/or DSTT menu.