for passive cooling, high density = bad, and im running passive for £30. if you can run oced passive using £30 of normal wc rads (as sold on ocuk)....
they do take up a lot of room. but look arround your pc - there will be a lot of room to put it anyway.
there are advantages and disadvantages to it.
pipe colour isnt nice, for example. and the bulk/weight.
but, truely silent running. (ever noticed how loud your keyboard is? i hate the whistle my hdd's make, and the creaks my mouse makes... ARGHHH)
cost is also down - unless you scrapyard yourself a heater core for £10.
Edit:
its been 55 mins, temps have leveled out, cpu at 58/57. room temp still 26c, water temp is now 34c. 50-57 = 7c, 28-34 = 6c
0.2662539939866349 is my system oC/w rating (
http://benchtest.com/calc.html) radiator is managing to shed 116w over a 8c gradiant = 14.5w/c assuming i had a quad, at say 150w and dual gpu's in the loop at 75w each... id have a water temp of 46c... less, infact as the rad will dissipate better as temps rise.