You can't directly replace the tubes with more powerful tubes (although you should upgrade the tubes to have some plant growing tubes and put reflectors on anyway). There are a couple of fairly easy solutions.
I have a Rio 180 sized tank (actually an Eheim, but same dimensions) which had 2 tubes and I've increased it to 4. I've bought strips of wood from B&Q about 2 inches wide 1/4 inch deep and about 6 foot long for under £5 (the cheapest ones I found have embossed cats and dogs down them and are meant as decorative rails I think, cheaper than just the plain wood). Then just cut the wood, so that it just fits and slides along the rails at either end of the tank like the existing tank light fitting.
Then.. buy a lighting controller (Arcadia is better than interpet IMO), 4 foot tube and reflector. The reflector has a neon tube clip with it and bolt, so drill holes in the wooden strip and use the reflector fittings to mount the reflector and tube (you may need slightly longer bolts than are supplied).
Alternatively, look at the T5 light fittings, which give more power from smaller bulbs and do a similar DIY job to fit them.
You may find the heat from the tubes affects the water temperature, so just take the plastic flaps out of the hinges and lay them on top of the tank instead and leave a bit of a gap at front and back to let heat escape.
I would avoid overdriving your existing tubes. Although some may recomend it, I know of one case of it causing a house fire.
