Everything Ramage said.
Main thing is... do you want the table to be portable? If so, you'll need to get a folding table. Be very careful when putting big dogs on folding tables--make sure the legs lock when the table is up and that the table is on firm, level ground, as they have a tendency to wobble and buckle.
I am a professional groomer and wouldn't be without my adjustable-height tables. I have one electric and one hydraulic. Personally, I prefer the hydraulic because the electric ones move sooooo slowly, but if you're not working in a busy salon, it probably doesn't matter.
They also have tables that are stationary and adjust by means of holes and pins in the legs. These are good if you have different size dogs, but you can't adjust the height with the dog on the table like you can with hydraulic/electric.
Lastly, there are fixed-height tables. They are cheap, light, and easy to move. With big dogs, they can get wobbly, so again make sure you have a good level surface. The obvious disadvantage is that you only get one height, but if you prefer to work at one height anyway, it doesn't matter so much.
Pet Edge has the best deals on tables, but shipping can be high, depending on where you live.
Grooming Tables | PetEdge.com
You'll also need a grooming arm and loop. Some tables come with a built-in arm, and others use clamp-on arms.