[shrimpchips]

Go with 2x 6 bulb fixtures - your tank is only 22" front to back, and only 18" deep. With a 6 bulb fixture, with the smallest reflectors, you're directly covering 12" (roughly 2" per bulb with reflectors), and at a depth of 18" you'll get pretty good coverage.

If you keep the rock work off the back pane of the tank, and assume a clearance of at least 6" from the front pane to the front of your rock work, you only really need to directly light an area less than 16" wide, which at a distance below the 12 inches of width of the fixture, should be easy to directly cover.

That, and you'll have less bulbs to buy each time and less fixtures to deal with.

Edit: If you go with something like a TEK light, where I believe the reflectors are 2.5" wide, you'd be getting a 15" front to back covered. Pretty much your whole rockwork will be covered directly by the fixture, which will be sufficient for most (if not all) SPS (with careful placement), and you can do the LPS and others lower down.