It could be a memory stepping you may have to change or it may have just needed a little more juice on the voltage. I can go to about 2.63Ghz (2630Mhz) before I need to start raising my voltages from 1.35v.
I wonder how much more power is in these cards????. Great jump from what you have pushed it so far.
Your temps are fine btw

. As long as you keep your eyes on the temps then you will be ok so don't worry when you get the CPU going a little bit faster and things start to heat up. As long as the CPU stays under the 70'C in CoreTemp then it will pass dual Prime / Orthos or whatever stability program you decide to use.
When you hit a wall, increase the voltage by just one increment and then reboot and see if it helps. If that doesn't then try one more increment of voltage and if that doesn't sort it then it's probably going to be the memory. Some motherboards "Auto" settings have put my memory to a timing that was causing it to crash in BIOS just after POST screen. If working on the RAM settings works for you as it did for me then just put the voltage back two increments and keep pushing faster. When you hit a wall it's more likely going to be voltage before RAM so when instability hits the further you push you can eradicate that problem more quickly.
As for RAM as you see my DDR400 2-3-3-6 1T is running at DDR466 2.5-3-3-7. I was having boot problems after I raised the speed and after playing around with all my settings only raising my latency from 2 to 2.5 got it started. I thought this was it but a few times my PC crashed so I ran memtest and errors should quite quickly with 8 passes Test 5 and Test 8 (Detects them best). I had another mess around with my ram timings and changing the last timing (TRAS) from 6 over to 7 and it's been fine ever since. I can get to just under DDR 500 with the same timings also but I only do that for benching.
This is just a quick guide to get you in the swing of things. Not to follow word by word as I'm I've had a herbal cigarette or two today. Oh and for Air I'd recommend 1.55v for the CPU to be your maximum to go to. Only increase the voltage on the CPU when you need to and only up until 1.55v then that's the max I'd go for air personally.