As many lumens as you can..you can get the 1750lumen (27w) cfls at homedepot/lowes or look for some flourex lights they put out around 8000+ lumens .
I got 2 cfl's from wal-mart they are fluorex sunlight bulbs that put out 1152 lumens and use 20W only 8 bucks
There are some realllly amazing cheap cfl's at some place for 1$, i beleive they only give out 1000 lumens.. but if you throw a bunch into a pie tin ( or anything for a reflector ) itl add up and BANG! If moneys not an issue, check the enviro-lites at a hydro store, theyr super high wattage and will penetrate better. Just gotta be sure that u keep them as close as possible to your plants