I wouldn't bother as P35's tend to run quite cool in my experience.
That said, most aftermarket coolers and waterblocks are quite flexible so may infact be compatible with any board that you later upgrade to.
I use a polarflo chipset block that was compatible with my old 680i, P35 and is presently on my X38. The noctua NC-U6 air cooler i had before was just as flexible and would also have fit on all of the above... including my even older P965 DS3.
Are you experiencing heat related problems at all?
gt