Watercooling problem

I have an core I7 920
Cooermaster cosmos s
2 ATI 5970 in crossfire
1200w coolermaster psu
8gb ozc triple channel pc1600 ram kit

I want to make my own water cooling system and i did some reaserch,
So is it better to reassemble my rig in the water tank or to assemble it outside of the tank and carefully lower it into the tank.

and PS : should i also put any external harddrives that might use in the with the pc ot should i just leave it outside.