Hi,
thank you for the information. It gives me a better idea on what we have to work with.
Firstly I just want to mention about you overclocking the gpu using catalyst control centre. It wouldn't surprise me that the reason your system locked up wasn't down to you overclocking the card too much (although that certainly is a possibility) but it was heat that made the system freeze.
Heat is the enemy and the cooler you can get your system to run the better. Remember that if your going to start increasing clock speeds (which should always be done with small steps) that also means an increase in heat and you should always keep an eye on your temperatures. Fortunately with catalyst control centre you can control fan speed too so always make sure it's increased with any type of overclocking.
Guru3D have an article on overclocking the 7970 and is well worth a read:
Radeon HD 7970 Overclock Guide - Introduction
The same goes with your CPU. Faster speeds mean more heat and again this must be monitored. Make sure the cooling is up to the job and if it isn't back off until new cooling is introduced. A new heatsink and fan doesn't have to be expensive.
Try to read up as much as you can about the hardware you own. I can't stress how important this is and being armed with lot's of knowledge will ensure a better chance of success.
I did find a guide on your cpu and hope it's some help, follow the links posted in the thread found here:
Guide to OC'ing i5 650!!
An excellent web site for overclocking in general is here:
Overclock.net - An Overclocking Community
As a general rule always try and get good airflow (whether your overclocking or not) around your system. Try to tidy cables out of the way so air flows better. Here's a couple of snaps of my pc to show you what i mean:
Always make sure your drivers are the latest versions and that you bios is up to date too. Regular cleaning with an app like Ccleaner:
CCleaner - Download and defragging the hard drives certainly helps.
Lastly perhaps one of the cheapest updates but one which will give you a big boost is an SSD, have you considered purchasing one?
Or you could turn two old drives into a RAID 0 device and get some free performance.