from what ive heard you need about 2500 lumens per sq ft or growing space, although the more the better I think. Im going to grow with these CFL's like 3 of these http://www.1000bulbs.com/product.php?product=4013 and
3 of these http://www.1000bulbs.com/product.php?product=4010 maybe more... but thats a little different then the tubes. You should still need the same amount of lumens though.