yes water cooling gives you a helluva lotta leeway when overclocking. I suggest Koolance, which although can be a little pricey at times (don't buy from koolance.com site!) it is my favorite system. Buy the exos system, cpu, gpu, optional northbridge (if it tends to get hot), and hard drive (same reason as northbridge).
for overclocking nvidia video cards. The most common way of ocing nvidia video cards is to use coolbits.
http://oc3dmark.octeams.com/tip02.html is a good howto on that.