Ben
It depends on what size batches you want to make. I aim to have 22ltrs after the boils, so after loses to the kettle, shrinkage after cooling, loses to the fermenter, I'm left with just enough to fill a keg. This means starting off with just over 28ltrs in the kettle and a 2400w element does this nicely. Anything less and it would take too long to boil and you would get a good enough boil. Any more an you run into problems with triping fuses, and the need to regulate the power once it has come to a boil.
Like Roach I use an immersion element or "the rod of power". What I like about these is you only need one, because you can put it in either your HLT or Kettle, and there is no need to drill holes and make bulkheads.
Cheers
MAH