probably have 100A service (or 60A, in my old 1940's house). I'd hate to see what that load would do on 60A - you would minimally expect your home electronics to burp every time the greenhouse heater kicks in - I bet it would destroy my TIVO. Figure a couple thousand dollars to upgrade to 200A (after the power company charges you for the new wires, and the electrician charges you for the installation, pay permits from the city, and you decide to go ahead and bury the wires since you are mucking with it anyway, and.... anyway, I've been there already.
Twice).

Do you have a source for oil fired greenhouse heaters? I've only seen propane or gas, but I haven't looked hard. Since I already heat the house with oil, it makes sense to heat the greenhouse the same way.
I'd need another oil tank, but that isn't too expensive. For what it is worth, my furnace guy says that warm oil burns better than cold, so if you have the option you should put the tank in a heated space. I don't think I'll be putting a fuel oil tank in my greenhouse, but I thought
I'd throw that out there.

Does anybody use an oil fired boiler for hot water heat in the greenhouse? I'm thinking hot water heat is the way to go for me. You can probably make a serious dent in your fuel bill by putting a solar hot water collector in line with your oil boiler. It is cheaper to heat hot water, and even on a cloudy day the solar water would be at least somewhat warm... Actually if you put a solar hot water collector in line with one of those 'on demand' electric water heaters, you might be able to heat a greenhouse with electricity for an almost reasonable price. You can build a solar water system from spare parts and your neighbors garbage. Or at least my neighbor's garbage (I see an old hot water tank out there right now).