I would look at the AMD 6870 cards you can get them for around 150.00 with rebates. I picked this http://www.newegg.com/Product/Product.aspx?Item=N82E16814102948 card up for a recent build for a friend. He couldn't be happier!
I bought a open-box ASUS P8P67 non pro for 104.00 dollars. Best money I ever spent. Can OC my 2500K to 5ghz but running 4.5ghz for 24/7 use. I say go with an Asus board even a cheap one. MSI isn't bad either.
I would undo the overclock on your second system and test with everything bone stock and see if you still have any issues. Then, I would overclock the processor (with your old OC settings) in 100mhz increments and test. What heatsink are you using on your second system? Are you using any...
Can you give a little more info? What are you playing\doing when the system crashes? Are you running any kind of video card software like Fraps, Afterburner, Trixx? What driver are you using? Are you overclocking? have you flashed your card with a new bios? Did you use RBE to modify the bios?
I have the catalyst 11.8 (Driver Packaging Version 8.881-110728a-122939C-ATI) but my powertune only goes to 20%. This is on a Gigabyte ref design 6950 (unlocked shaders). Which Catalyst drivers have the Powertune with the range of -50% to +50%?