my guess would be 2 40w tubes... you don't really need 'plant' bulbs. my preference for tubes are the GE Aqua Rays - Fresh & Saltwater tubes. they have nice color and aren't as 'yellow/green' as some of the other bulbs out there. my plants grew fine under them.
im pretty sure you have 2 15 watt bulbs, barely enough to keep low light plants although i have a big carpet of java moss in .75 wpg in my 20 gallon tank.
My 55 came with 2- 15 watters. It was almost impossible to grow anything that didn't float at the top. If you want decent plant growth, UPGRADE! You'll be glad you did.