I have a cage that is 5'L x 4'H x 2.5'W and I only have one basking light. But I do have 2 4' double fluorescent fixtures across the top. Three of the four bulbs are 5000k daylight bulbs by Philips that help brighten up the cage and give my plants a boost to grow) and one is a Reptisun 5.0. This actually helps my cage stay a little warmer across the top and then keep the bottom more room temp (73-75*), so there is a slightly better gradient.
You don't have to use as many lights as I do, one double fixture would be more than enough. But I had a second left over so I put it to work.