I run SLK-800's with Panaflo L1A fans on them on two of my duallies. They cool nearly as good as a watercooling system. You can save $10 on the heatsink if you get the SK-7. It is very close in performance to the SLK-800, but costs $20, instead of $30.
If that is still too much money and you are looking for the absolute best cooling bang for the buck, get a SVC GC68 from
www.svcompucycle.com. They are only $6 and come with an 80mm fan. It is a REALLY good heatsink for the money and fairly quiet. I use this heatsink on about 40 rigs, although I usually replace the stock fan (pretty quiet itself) with the slightly quieter Panaflo L1A.
If you don't already have case fans, then I suggest either the Panaflo L1A or PC Power & Cooling Silencer (slightly quieter than a Panaflo L1A.) Of over 30 fans that I've personally tested, those are the quietest. The Silencer makes a great case fan, but not a good heatsink fan. It doesn't seems to be able to push with enough pressure for heatsink use.
*EDIT* I have a Vantec Stealth and it is not as quiet as the Panaflo L1A or the PCP&P Silencer. It may be rated lower, but to my ears, it is definitely louder.
*EDIT2* Usually the fan(s) in a PSU are quite noisy. You can replace the noisy fan in your PSU with a new quieter fan, too. I usually replace the PSU fan in all my PSU's with Panaflo L1A's wired directly into the 12V line. I could hook them into the thermal controlled ouput in the PSU, but I prefer to wire them into 12V so they help with case ventilation. Either way is good, though. Most PSU's with two fans can run fine with just one, too. The one's that have a fan on the bottom are terrible noise makers. The bottom fan's airflow causes disruption in the rear fan and causes quite a bit of noise. I just take the bottom fan out and put duct tape over the big hole, making sure, that now, air comes into the PSU only from the side opposite the fan. This is prime airflow for the PSU, since it MAKES the air go over the heatsinks inside the PSU.