Is your tank a 20 High or 20 Long?
The 55 watt bulb is around 24" long (with the end and the socket all attached), where as the 36 watt bulb is around 18" (with end and socket)
From my AGA catalog, a 20 high is 24" long ... and a 20 long is 30" long ...
so if you have a "long", then the single 55 would be fine ... giving you moderate light of around 2.7 wpg
if you have a "high", then the 55 won't fit, and you'll have to go with 2 36 watt bulbs, which will give you moderate-high lighting at 3.6 wpg
Fishfreek is correct, if you buy the "BrightKit" the reflector is included.
AH Supply's prices are very inexpensive compared to what a commerical fixture would cost, however, they are not "bottom dollar" priced DIY
components. You have a premium for buying the pieces in a "kit" form, rather than buying them all separetly, sometimes from multiple vendors to get the absolute lowest price.
If you'd like more help on a true Do It Yourself lighting project (including making your own reflector), let me know ... I've done it several times now!