The PCIe slot that your video card sits in can only provide 75w.
Most video cards also have an additional power cable or two coming from the power supply which can provide as much as 300w more power. There are two types of cables that can come from the power supply. The first is a 6 pin connector, which provides 75w. The 6 pin connector can actually use an 8 pin connector which leaves 2 pins over-hanging the end of the 6 pin connector. If you look closely, you will see that each 8 pin connector has 6 pins that are a solid piece, and then 2 pins that can flex outward on one end or the other. Those 8 pin connectors provide 150w when all 8 pins are in the 8 pin socket, or 75w when only 6 pins are in a 6 pin socket.
If a video card has power plugs on it that needs cables from the power supply, most of the time they will say something during bootup on the screen, and then stop the boot process until you connect the proper power cables.
Now, if your card has no power cables, and you over-clock it to where it needs more than 75w, then your video card and possibly your motherboard will either:
1) Power itself down
2) Shut the system down
3) Become unstable, eventually crashing the system.
4) Some unpredictable scenario that could damage hardware.
Bottom line? I would not over clock a video card that did not have power cables attached to it.