I rate the sandisk extreme card range got a class10 in my d3100 and a class 6 in my little camcorder thingy, both 8 gigs
http://www.amazon.co.uk/exec/obidos/ASIN/B0014VY28A/ref=ord_cart_shr?%5Fencoding=UTF8&m=AFY1UKFQTOIA6try these. reliable and fast enough
Currently unavailable.
You might find you'll not have enough battery for 40 min of filming. They chew up the juice...
in testing some class 4-6 cards outperform class 10.
although I realise you likely have little intention of filming continuously for 40minutes, you might have a setup going for more than 5..