You could possibly (dependent on many factors) hit 2.6-2.9 on either. The 4400 will be easier to hit the higher numbers with. On water cooling you could hit just a tiny bit higher then on air. Assuming your using a good wc vs a good air hsf (also assuming your not doing some kind of ice/slush cooled area for the tank to sit in instead of active cooling).
Water coolers are funny...If you're cooling both a cpu and the nb then your water will be running hotter then it would just on the cpu...Likewise if you're trying to cool a gpu in addition to your cpu. Then you want to look at more tanks, a larger overall one, or preferably a second water system... If you get a crappy internal wc, you'll see the same temps you would see with a avg hsf. External water cooler with active cooling is the way to go if you're trying to keep temps down. There are always a lot of factors to look at...For instance if you're not running air conditioning...yea...that is going to adversely affect your higher oc regardless of water or air (again assuming your tank isn't submerged in slush heh).
Anyway, if you're trying wc, buy a good brand, or put one together yourself (there are a few guides on how to put one together yourself out there:: i'm a fan of using a heater core from an '80s Chevette myself...but i haven't tried it myself). Additionally, if you decide to go with some type of wc...do not test it out for the first time on your hardware...test it outside to make sure it doesn't leak.
I have an X2 4000+ maxed out stable at 2.85GH with enhanced air cooling (Zalman 9500). Use the speed primarily for gaming and video processing.
With the low cost of these CPU's it's worth the risk to get every Mhz possible.
I have taken my Brisbane 4000+ X2 all the way to 2.9GHz. I firmly believe it will hit 3.1+GHz (or more) stable, given the rest of the system is capable of handling that speed.
The only reason my OC hit a wall at 2.9GHz is because of my RAM. Right now I have the clock set at 2.8GHz and the RAM will not break loose from the lowest divider.
FYI. My rig is externally water cooled with dual 150mmX200mm radiators that use 140mm fans. The pump pushes 500+GPH and 6 feet of head. I will be taking this little Brisbane chip to it's limits when I get some better RAM.
My 4800+ Brisbane 3.125Ghz on air @ only 1.41 Vcore and i haven't seen any other 4400+ even close.. The 4800+ @3Ghz (witch is easy to achieve) is equal to the 6000+ on performance. So I would say the 4800+ considering the prices..