If you're mining with even one decent card you'll want to use a radiator regardless.  The heat dissipation through a flat surface (container wall) just isn't that effective.  There's a reason every piece of computer hardware uses fins.  If you use a glass aquarium than you will dissipate some heat.  If you are using an acrylic aquarium or plastic tub then you will lose less heat through the walls.  Having a pump attached will help circulate the oil which is also a good thing; relying on natural convection for your fluid circulation will raise the temperatures.  If you're using multiple cards than you will certainly need additional cooling.

Both my mining and gaming rig are in oil, but I would recommend against it unless you have some idea what you are getting into, and you enjoy that sort of thing.  Puget systems does have a good kit and explanation, which makes it much easier.  I tried their kit some time ago, and decided that the aquarium they were using was a bit too fragile.  However, the oil is not cheap (~$20/gallon), is very messy, almost certainly voids warranties, and still requires additional cooling equipment.  Also, your hardware is difficult to get to, and your computer becomes damn near unmovable.

If this is the kind of thing you enjoy doing, then by all means, do it.  But don't get into it thinking its going to be cheap and easy.