it depends greatly on what kind of cooling you have.
if your running on air then your probably going to hit the wall at about 3.6ghz-ish no matter what you go with.
with water you might be able to get to 4.2ghz, if your psu is going to run out of power first is a different story all together.
if you go with the almighty phase change, then i have a question for you, what are you doing with a Pentium D 915?
i only have limited knowledge of that particular cpu though, so im baseing those numbers on various other pentium D's ive seen overclocked.
I would be running on-air, using Scythe Mine Rev.B HSF. I have currently got the 915 running at 3.8GHz.
Anyone else had experince with this CPU?