Money is a relative thing - if you plan to one day buy any corals that require more light than your fixture can deliver, then you may have to upgrade again or supplement it with another unit to increase output.....Most SPS and some LPS may not do well with an output below 4watts/gal and prefer somthing above 5. Nothing worse that spending hundreds on a bunch of SPS or other high output desiring corals and watching them waste away under insufficient light.....A tank deeper than 18" usually requires ALOT of light and metal halides do about the best job in depper tanks - or multiple VHO units....Also remember that any salt accumulation on a light unit lense RAPIDLY reduces output...
It's just a matter of what your future plans for are for the tank - but If I wanted acropora, seriatopora, stylophora, or espically Pocillopora - i'd probably suggest ALOT more light....I run twin 250w HQI and more than 250 watts compact florescent over a 100 gal tank - over 700 watts worth - and i'd not run anything under 500 in my tank and expect much of my SPS collection to survive...