The reflectors will be a big part of whether you can get away with 250w or if you should go to 400w. If you're wanting to be keeping that clam on the sandbed, I'd be looking at 400w. If you weren't sold on adding SPS though, definitely 250w would be enough. Most people like the radiums on tanks like this, but the bulbs are expensive, and don't burn as long as a lot of other bulbs. You could grow SPS anywhere, even with 30" deep though using a radium, so there is a benefit to it, and the color of their bulbs are terrific. With your current stock list, you could probably even get away with upgrading the reflectors (if needed) and just using the 3x250w halides to grow LPS and softies, even in the larger tank. It wouldn't be ideal, but without having plans for an SPS dominated tank, I don't see the cost of adding another halide, or changing out the whole lighting system as being worth the expense. That's my opinion.
