I have an asus K8N-E Deluxe mobo, I had to rma my graphics card which is an ATI AIW Radeon 9800 Pro. While my AIW was gone I used an old TNT2 16MB card, I do not know the voltage on that card. When my rma'd card came back I installed it and the system will not boot, post or anything and the hdd activity light stays lit up. I tried the AIW in sons pc and it works fine, I then took my sons ATI 9800 Pro and put in my pc and it worked fine. I am completely stumped. I have tried clearing the cmos, disconnecting other hardware and running power straight from psu. Any suggestions are greatly appreciated.

Sounds like the 9800 is not compatible with your motherboard's AGP slot. Check the following site for some info:

AGP info

Well, since the first aiw 9800 pro worked in my system and sons 9800 pro works in it, I don't see how the mobo is all of a sudden not compatible. I talked to ati this morning and they said the card they sent me has a newer bios on it than what my older card had, so they are gonna rma me another aiw 9800 pro with the older bios.