I found a 40W one as well...

http://www.homedepot.com/Electrical/h_d1/N-25ecodZ5yc1vZarcd/R-100117845/h_d2/ProductDisplay?catalogId=10053&langId=-1&keyword=plant+lights&storeId=10051

That 20W is 600 lumens...you can get double that in a single $8 CFL. Two 40W seem decent (3200 lumens for $11). Enough to start some seeds. But there are more efficient setups available at Home Depot such as a T8 or T5 which generally gives you more lumens per watt. There is nothing particularly beneficial about an "aquarium" lamp -- just look for most lumens for the least cost.

According to this Seed FAQ on this forum, it says lights with warm spectrums would be beneficial in some bits, and these aquarium/plant ones seem to be the only ones with warm light... perhaps I can use one of these and put a different more efficient light on the second shop light socket?

http://faq.gardenweb.com/faq/lists/seed/1998034312029084.html

I would say use daylight fluorescent bulbs (T8, 40W) or cool white or a combination of both in each fixture. I would not waste my money on specialty lights, especially since both give off a fairly wide spectrum that will support plant growth very well.