I just got an Acekard2 myself, and to be honest I haven't had any issues with an 8GB MicroSD card. Mine's not even a well known brand (It's PNY, which I have heard is a re-brand of Kingston.), I bought it last year when I got my 1st Flashcart (M3DS Real, which has now been relegated to "use only if push comes to shove status"). It's a Class 4 too.
You'll probably wan't to get at least a Class 4. Class 6 is even better if you can obtain one. If push comes to shove you can get away with a Class 2 but don't expect fast loading speeds or compatibility with games such as Castlevania: Portrait of Ruin, Castlevania: Dawn of Sorrow, and some other such games.
Come to think of it DOS and POR both still freeze on my Class 4 MicroSDHC, so if you aren't going to play those games you can probably get away with a Class 2. You're still better off with a Class 4 or Class 6.
4GB fills up pretty fast, of course depending on how you use it.
Keep in mind my Acekard2 is not a AK2i. It's only the standard one. However I would think the same concept would still apply to the AK2i.
I'd say go with Kingston, 8GB and either Class 4 or Class 6 (If you can shell out a little more for the Class 6, then by all means do so. Otherwise a Class 4 will usually do fine.)