I dont know much about the ol MX's cept they suck. You can overclock the video card's core and memory using Rivatuner or powerstrip
www.guru3d.com is where you can find rivatuner.
Just go slowly on the core and memory going up by 5mhz for each then testing the card by playing some games and running 3dmark benchmark's. Keep going until you get artifacts or the card lockups or reboots. Then slowly go back down in speed until it doesnt artifact and reboot anymore.
Video card sensors only came out until recently so yes its perfectly fine to oc without one.
Btw welcome to the forums.