Well to jump back to the topic at hand, i personally believe that 2GB is more than enough.
it really depends on inamo, how much do you think you will need and why? that is, what do you plan to store on your microSD, be it games, music or videos, programs whatever.
Personally i have a video ipod to serve my video and music needs, i plan to get an R4 in the coming weeks solely to play games on the NDS lite i plan to buy. That's all i'll be using it for, so 2GB is more than enough.
If you believe you need +4gb for 4x the price, then you'd have to wait a while for it to be a reasonable transaction, however if this is just for gaming then i'd recommend going for an R4 or M3DS.
For me, 2gigs is no longer enough. I keep popular games on my R4 that I personally don't really like just because some of my friends do like them. It's nice to have a vast variety of games in one cart just to show other people who may not necessarily like the same games you do.....especially in my case where I have vastly different tastes in games from everyone else.
At this point, 4gigs would be plenty, but my 2gig card is constantly filled to capacity and SDHC compatibility would be really useful to me.
What I want to play today may not be what I want to play tomorrow, or the next day, so having a variety of games to choose from is a must for me. Some days I want to play something simple and quick, other days I want to play a more in depth game. My mood changes daily.