Look at the sizes of the MK3's included,stuff like that.
There are a lot of numbers involved in NEO products.
It really depends on what you want to spend,though.
Dr.Neo is working(the menu releases)on decreasing the hardware involved,but for now,the 1g Neo-Cart(comes with a (relatively)useless MK1)[Divineo CN is cheapest on that(fast shipping)]about 130.00,and the MK32006(divineous)about 90.00,will provide the highest compatibility.
I have a couple of set-ups(a LOT of NEO stuff)my son uses one,I use another,but this is just my opinion.
Speaking frankly,if your just a dude(or whatever)that wants to do fun stuff with your DS(like me),not a coder or anything,then NEO probably is right for you(reflecting on the thread title there)