Zalman is a very good brand and they making very good products.
I was using Cooper 120 mm Zalman kit in the past and I have to say that I was impresed.
The temperature on my overlocked Athlon 3200 + decrease from 55 to 44-48 degrees on 100 % CPU.
10 degrees is one of the best scores I'v ever seen.
And the games speed up about 2 - 5 % (fps tests)
Bigger Zelman = bigger cooling surface = lower temperatre = quieter fan