I think the R4i is the only popular card which hasn't been updated for firmware 1.4 at the moment although this could have changed since the last time I looked. I personally have the Acekard 2i and it works but I so far haven't liked it nearly as much as the G6/M3 Real I had on my original DS. There seems to be a lot more upkeep in order to keep it current and working. Given a choice now I'd probably read up on my options a bit more, I'm just not fully satisfied with the Acekard 2i.
Also be sure to apply the 1.4 patch to your card BEFORE updating your DSi to 1.4.