If you already know how to overclock I'd shoot for 10%, 15% at the outside. You also dont have to get water cooling, you can buy a very good air cooler for 30-40 bucks. I use the Big Typhoon, and its awsome. (it does weigh over a pound and towers over the rest of my case, but its great). I'm running my x2 4800 at 2.7ghz, stable, and I plan to push that higher when I get some time. 45C under load at 1.45v.
If you're going to use the stock cooler you probably wont be able to get much more that 200mhz more than the default clock. And even that will probably have you running on the hot side, temperature wise.
