That heatsink doesn't look too much better than a stock heatsink. Look into getting something like the SLK-900A that's what I have and it works great. Doesn't come with a fan though, but coupled with the 80mm or 92mm Tornado and you'll have plenty of cooling as far as the CPU goes, the tornados are quite loud though so a fan controller is recommended. 55C is about normal especially considering you've even OC'd that chip a bit. Like I said, my 2400+ XP (not the mobile I currently have in here now) would run at 64C idle when I had stock heatsink, no case fans, and the side panel open, so I think your temps aren't anything insanely out of the ordinary, but most people would be a little more comfortable if it were in the 40's.
If you REALLY want to OC I suggest getting a mobile chip like this. The 35w 2400+ XP-M through the 2600+ XP-M's are supposed to be great at overclocking and at cool temps at that. You gotta figure it's probably in the 60's celcius in a laptop so it's real easy to cool it down in a desktop with adequate cooling.