It brought it home to me, just how vital case cooling was, a couple of years back, when I too started to have issues with crashing etc on a severely clocked PC.
I did almost everythign I could to improve the air flow with what I had, and ended up using 4xfront 80mm 2xside 80mm and 2xrear 80mm and the CPU another 80mm and all these were all deltas, so its not like they were not powerful, but in the end, the best cooling solution I found out, was to simply not skimp on the case.
I bought a Thermaltake Armour, and my temps dropped to 36c using the supplied 120mm fans, and I never looked back since.
I later bought a Coolermaster Stacker, and found that this didnt give quite as good cooling as the Armour, but to be fair, I did use the cases for differece systems, so I cannot say for sure, but that certainly cured all my cooling issues.
Ok, the CPU Cooler I used was, now dropped to an SLK700 or an SK7 with a cheap and nasty 80mm UV Fan, which is only 18CFM, but the case cooling was so good, it was better than using a Delta80mm in my other case.
So, anyway, you dont want to spend much on it, so how about doing somethign that should cost only pennies...
Take the PC apart, and clean out any dust from around the HSF and anywhere near fans etc... Also, re-seat the HeatSink onto the CPU as that can often gain you a few extra C's