Possibly, but the slot fan is much larger in diameter than the gpu fan, so at the middle of the slot fan its not drawing any air in, it draws air from outside the gpu fan, sucking in the air that just blew off of the gpu. I think its helping more than its hurting.
Well the best thing to do is check the temperature of the card when you get it both with and without the PCI slot fan. This will let you know whether it's working well or not.
As far as having to buy aftermarket cooling, you won't need it really unless you're going to overclock it.
Well my 9800pro doesnÂ’t have a temperature sensor. So I really donÂ’t have any way to find out. The slot fan is in the pci slot two below the vid card. So there is a open slot in there between the slot fan and the vid card. There is a good amount of room and air flow. Plus I have an 80mm fan on the side of my case blowing right on the video card. This is a high amount of air flow. I should be alright with this current setup.