I'm using this very heatsink on my video card right now, and the performance of the Zalman is extremely good. I have an ATI 9500 128MB card that I've performed the modification to that makes it into a 9700 non-Pro (the mod is to move one resistor on the card and flash to a different BIOS). The default cpu clock of my video card is 275MHz, but I had replaced the stock heatsink/fan with a Thermaltake CrystalOrb and was able to overclock the core to 325MHz (anything over 325MHz resulted in video artifacts). With the Zalman installed, I run my core at 345MHz with no problems. Added benefit of the Zalman is that there is no fan noise (it has no fan on it). One thing to note is that the Zalman obstructs the memory on most video cards, so you may have trouble putting heatsinks on your video memory.
IMO, the Zalman is well worth $25.
E.