If you have the money to spend, the Vapochill system is worthwhile.
I correspond with quite a few Vapo users, and alot are running around 1.7Ghz and up ( some are @ 1800 plus "STABLE). This is with various CPU,s 1ghz,1.2ghz.1.33ghz
"STABLE being the key word here , you might attain similar speeds with air cooling , but most times air cooled systems at 1.6ghz and up, are not !!( i said most not ALL).
It also depends on how far you really want to push your CPU, if you want to wring every last mhz out of it , go for the Vapo, BUT if if this is NOT your aim then your money may be better spent elsewhere!.