If you want to use anything other than the standard flourescent bulbs that came with your fixture you will need to either modify your existing fixture or replace it. The ballast probably wouldn't even be able to turn on a VHO bulb even if it would physically fit.
Cheapest alternative is to ODNO (Over Drive Normal Output) if you do a search in the forum you'll find lots of information on how to do this. Basically you replace the ballast in your fixture and increase the power you are putting through standard bulbs. This gets you more light output, but also shortens the life of the bulbs.
The next least expensive option would be to DIY a fixture from scratch. Again a search of the forum should including the DIY forum should get you lots of ideas on how to get started.
For those that aren't into DIY or wiring, I would say that a PC fixture is probably the next least expensive option. There's a lot of good deals out there, although you may have to swap the bulbs as many are geared toward SW instead of FW planted.
