If I remember right about ATM from last time I looked into it the speed on you ATM interface is more like a max speed, so if the ATM card should support all ATM speedsettings from the cards max and down.
I think that I asked Unisys about what interface cards I should buy if I wanted to run 4 Mbps ATM. He said 155 Mbps (price was almost the same as a 45Mbps interface card)
I could be wrong, but that is how I remember it, but this was for a 7206.
/johnny