Hi leedsfan, welcome to itsallPC

From memory, the K8NHA grand is an nForce 3 chipset motherboard, meaning you have AGP. If you have a GeForce 6600 GT, it will require a molex power connector (Vanilla 6600s do not usually), unlike the PCI Express version that can draw sufficient power from the slot. I'm making this assumption since you're not particularly precise with your hardware specifications.

If this is so, double check that you didn't dislodge the molex power connector from your graphics cared when installing the new Hard Disk Drive. If your Hard Disk Drive and graphics card are sharing a power cable, plug them into different cables if possible. If this is not so, post back.