I think the biggest factor is the Dell case.
Pic 1 --->
http://megamall.co.nz/images/DB2/sto...S-R-Photo2.jpg
Pic 2 --->
http://www.2cpu.co.kr/data/file/hard...1000170_s1.jpg
That setup is not great in terms of unrestricted air flow. Plus the expansion slot section has only one fan feeding it and nothing to help evacuate hot air. It would be fine in an office environment but as a cruncher, not so much.
Normally I would suggest something like a HAF 912 case but Dell does too many funny things with components. So hardware like PSUs, MBs, etc. won't fit in standardized cases. Even Dell's fans use a non-standard 5-pin plug so you have to buy adapters if you want to upgrade fans, or rewire them...
The only other suggestion I have would be to get an aftermarket GPU cooler although that has it's own challenges........