I'm not familiar with the PAR rating of the MarineLand fixture you are currently using, but the new Finnex Fugeray fixture comes in a 30" width and is rated at 30 PAR @ 18" (a relatively common light to substrate distance for 30-50G tanks).
Here is a link to the PAR data for the different fixture sizes.
http://www.plantedtank.net/forums/showthread.php?t=183246
Here is a link to the PAR data for the different fixture sizes.
http://www.plantedtank.net/forums/showthread.php?t=183246