There are better coolers than Zalman at similar price, next time try to post you system, for example case and mobo would be useful atm to know if you have 4pin pwm header for the fan and how much clearance the case has for tower cooler.
For sub $50 i like a lot Scythe Mugen 3
(158mm height), comes with a very good PWM fan that can be drop down to 300rpm where its totally inaudible, but can ramp up to 1600rpm if you need the extra cooling (not silent at that speed), but 4pin pwm fans are controlled by bios/software so really depends if you mobo has 4pin pwm fan connector (check your cpu fan connector header if its 3pin or 4pin). Now if its 3pin, i would probably just go with something like COOLER MASTER Hyper 212
or XIGMATEK Gaia SD1283
and swap the fan with Slipstream 800rpm SY1225SL12L
. Or if you want the top of the line cooler for single rad, Prolimatech Megahalems Rev.B CPU Cooler
and add the same slipstream fan. Check the following reviews for more info,SPCR Prolima Megahalems: A Mega Nehalem CoolerSPCR Scythe Mugen-2 CPU CoolerSPCR Xigmatek HDT-S1283 & SD964 "heatpipe direct-touch" CPU coolersSPCR Coolermaster Hyper 212 Plus: Direct Touch Revisited
In case you cant fit 160mm height tower coolers (depends on the case), then look into Noctua NH-U9B SE2 92mm SSO CPU Cooler
(you need 125mm height clearance), check your mobo to see if you can mount it, memory and other components. It should come with 3 in line resistors that should allow you to chose the rpms of the fans.
Edit: If lodestar is right into you having the Asus P5Q, from what i see in newegg mobo picture, it has a 4pin CPU fan header,
so i would go with the Scythe Mugen 3
as long as you case can fit 158mm height coolers.