Leadtek actually has a better heatsink than the eVGA when it comes to the 6600GTs
Coconut, to answer your question, it's because eVGA makes very good quality cards. For example, there have been numerous posts about peoples XFX brand cards screwing up, but I can't say I've seen an eVGA.
They have 2ns RAM compared to some brands have 1.6ns so that may hinder OC'ing the RAM by a little, but after a week of burn in my card OC'd to 564/1109 from stock 500/900 (this AGP of course, PCI-E is stock 1000MHz memory)
The core can't go much higher due to temps pretty much, but it was still a good increase in performance.
Also you can't beat the step up program. I sent in my card two days ago and should be getting my 6800GT here in a couple more days
within 90 days if you have enough cash to pay the difference between the 6600GT and a 6800GT (its $399 at eVGAs website still compared to like $379 at a place like newegg)....anyway, pay the difference, send in the 6600GT, with ALL components and original packaging, and you can upgrade to a higher card.
Can't wait for mine