Phil's Computer Lab did a video on Voodoo2 cards, chip temperatures, and cooling. I watched it earlier today. What a weird coincidence.
I've considered using thermal epoxy to bond heatsinks onto all three of my cards (12 heatsinks in total), but I haven't had the nuts to do it as it would permanently cover up the chip's markings and those cool 3dfx logos on them.
If you have a side-mounted fan on your case, it's probably on an area that would blow straight down onto the cards. Phil showed that just having one nearby fan blowing cool room-temperature air (which your side-fan would provide) dropped the temperature roughly 20 degrees Celsius - WITHOUT a heatsink attached. That's impressive.
Having said that, if you think your temps will be a problem even with the side-mounted case fan, then go with the heatsinks. It will even further cool the chips to the point where you'll never have to worry about them again. But I've been running SLI 12MB Voodoo2 cards in my case with NO side-fan (the case doesn't support one) and I haven't hit any kind of thermal issues yet (screen tearing, bad textures, missing pixels, dropped frames, etc.). It's really up to you. My situation and PC setup is probably different than yours. I only have a 600MHz Pentium III on a 100MHz FSB, so it's probably not enough power to get my cards cooking that hot. Anything I have faster already has vastly superior graphics cards, such as the Radeon 9550, GeForce 7800 GTX, GTX 480 SLI, or GTX 980ti.