Well i used to have a Zalman VF700 on my old Geforce 7600GT, and i really liked it - temps were nice and low, and i couldn't hear it.
The purpose of the AC-S1 heatsink was never to break any overclocking records, although to answer the fan question, my understanding is that you can add these bad boys for a fiver:
http://www.scan.co.uk/Product.aspx?WebProductId=627856
They look like they'd take all of about 10 seconds to add, as opposed to the 30 minutes to get the heatsink on!
They look good, and perhaps i'll get some at some point, but i think it's a bit poncey calling them 'Turbo Modules'. They are just fans! Personally, i really preferred zero noise over improved cooling.
As for a comparison to the VF900, a quick google search gave me the ever reliable
[H]ardOCP forum with this post:
I had a hard time installing this thing (Those plastic L + T addons are annoying as hell! Kudos to anyone who managed to install them), but the temperatures are quite pleasing.
Beforehand I had a Zalman VF-830 on my card (Factory installed):
Idle: 49-50C | Load: 65-67C
Switching to my old Zalman VF-900 didn't help so much:
Idle: 47-50C | Load: 67-72C
With the Accelero (And the turbo module):
Idle: 45-46C | Load: 55C
The load improvement is the most important to me, so I'm quite satisfied with it
This seems to be on an 8800GT, so i suppose the 'turbo modules' seem to be good.
The turbo modules themselves are
reviewed here.
I would wholeheartedly recommend getting the AC-S1, and you can easily add the fans after if you feel that you need them. What case do you have?