I have had a slot cooler I got from best buy sitting under my geforce2 for a year now...and I finally realized that it wasn't really doing much, so I just modded the vid card, put a huge-ass heatsink from 3dcool.com for those old cellys and a pretty beefy 60mm fan, then stood a 120mm fan on the bottom of the case blowing directly at the ram chips. I got 10-20mhz more on the core and ram depending on fan speeds...
Also, i think if you just take one of those slot coolers and flip it over, (so the intake for the fan is on the bottom side) and put it ABOVE the video card or another PCI card, I think it would cool much better.
haven't tried it yet but logically since heat rises, this would work much better than having it below the video card...maybe I'm wrong though