I would suggest a 24" 4 bulb VHO retrofit kit. Each bulb would give you 75 watts or 300 watts total. This would allow you to keep a Bubble tip anemone and plenty of softies and zoos.
The 3 bulb kit would give you 225 watts and you could still squeek by, not much of a price difference though.
I like Bang Guy's idea as far as BEST lighting with the 250 watt Metal halide though.
Thomas