I can purchase the Leadtek 8800gt GTB and ZL for the same price. The GTB has the a slightly faster clock speed at 680mhz but the ZL has a custom zalman cooler and is clocked at 650. Although the website states that the ZL has a Zalman cooler for good overclocking potential, from what I saw Gigabyte also released a 8800gt with a custom zalman cooler (looks very different from the zl oneso theyr different) and apparently it actually performed worse that the Gigabyte standard 8800GT when overclocked, even though it ran cooler at stock speeds.
but its funny the better cooler has a slower clock too weird! i would go with the faster clock myself 680mhz
the last Leviathan system:
If the ZL with the Zalman cooler is any good then you may be able to overclock it quite well. If your not going to be overclocking then just go with the 680mhz, plus you say the ZL isn't a great performer even when overclocked.