Oh yes! You can certianly have clams in this hobby. Not the best of ideas in a small Nano but with proper attention to detail it can certianly be done. They are best and easiest done in tanks 40 gallons plus.
I use one source for these and one source only...
Clams Direct. Barry is located outside El Cajon California and has some of the most vivid and eye popping clams bar no one. I promise you, you will not find a outlet that will give you the kinda customer service Barry gives out either. His prices are about as reasonable as it gets as well. If there is someone better please post a link! Barrys clams are also WYSIWYG and I promise it will look just like the picture when it arrives.
Clams Direct
Some of the better clams to try are Maximas and Croceas. The other while nice usually arent quiet as nicely marked and colored but more importantly if properly cared for will get quiet large. I would suggest that you get a clam thats at least 3 inches in size. These can usually get enough food/energy from your lighting if its adequate. Smaller clams than this while cute will most likely need to be taget fed. This is best done in a seperate contianer as pouring something like DT's into your tank can quickly over whelm it and send water quality south in hurry. Not to mention get expensive fast!
Clams are at their best when kept under some intense 10,000K to 12,000K halide lighting and placed in the tank where they can be viewed from high up above them. This is where they will show thier best colors. In small tanks you will have to watch the Clacium levels as they can be quiet the Clacium hogs. You may also have to add small amounts of Stronium and Magnesium to smaller tanks as well as you might not be able to keep up with a growing clam on these two by water changes alone. In a bigger tank these two are usually not an issue.
Clams are awsome and can make for a great center piece in your tank if properly housed and cared for.