Your temperatures are pretty good, especially with the 55c on load. My CPU is a 65nm 5200+ at stock speeds and will upgrade as soon, with a Zalman 9500 cooler (will upgrade this one soon too), I never tried it with the stock cooler though. Temperatures are worse than yours but then it is quite hot and humid here right now.
For heatsink coolers, my preferences are the simple installers because I tend to open my computer casket every now and then and do stuff to it, I don't like taking out my mobo anymore (~_~). Xigmatek models uses the stock AMD brackets, same with Akasa models. And the temps on most of those models are cooler than my Zalman, and they emit less sound decibles as well.
Ideally it is best to have a perfectly flat base for a heatsink cooler, but CPUs aren't flat (they are slightly concaved for several reasons), Thermalright made several of their heatsinks slightly convexed but models that are convexed performed slightly worse than flat ones. Maybe after mounting it, gravity or heatsink paste or both affected the heatsink position slightly, dunno never looked into it. For me, best stick with a flat base.
Check out Frostytech.com for more on heatsink stuff. | 
