I chose the 6600 as it is the cheapest currently available with the 4MB cache.
Otherwise the right choice would probably be the E4300 - the cheapest Conroe.
I blogged my Zalman Reserator V2 setup here. Only thing of note is I've got a small very quiet housefan blowing on it now which helped it by 5C or more. It used to be quite warm - now it stays much closer to room temp. The fan really helped.
FSB: 390
VCore: 1.5
Idle temp: 31C
Torture test CPU core temps: 56C
Northbridge temp during torture test: 41C

On water, motherboard temps are higher at idle and lower at load - I assume because
it is getting somewhat warmed water as it is second in the series after the CPU.
Use CoreTemp - The Asus MB utility under-reports CPU temps by 4C on my system at idle, and 6C at load. Coretemp is known to be more accurate, as it reports temps reported by the package. I believe the Asus utility is reporting a sensor on the motherboard somewhere.
I also use Prime95 for my torture testing. I use two instances of it, one bound to each core to make sure I get both cores to 100%.
I've had a bit of trouble with the Asus P5WDH Deluxe northbridge temps.
It has a complicated (but nifty) heatpipe cooler for the northbridge. It comes
with decorative covers on the heatsinks (Come on Asus - that's silly). After removing
those and fiddling with the seating, my northbridge temps got to something more reasonable
(hovering around 40C instead of 50C).
Still, with the new processor in hand I decided to do the job right this time and water cool not just the CPU but also the northbridge. I also decided to get the cover off the southbridge, clean it, and get a properly applied heatsink compound applied. Photos below...
Southbridge before (with silly cover):

Southbridge after cover removal:

Northbridge before (silly cover already removed):

Northbridge with heatpipe/heatsink removed and ready to go:

Final setup (those big air gaps will disappear once I hook it up to the pump again):



I assume you are also cooling the SB. I've read that P5W DH has its temperature sensor near the SB and that the system temperature is actually that reading.
Also, it would be useful, if you could share your BIOS settings on the blog.