You're getting a Dell.
I know you mentioned you already tried this brand and they overheated however I think I can direct you in the correct direction. Disclaimer though I am a huge fan of Dell and pretty much use all their products (enterprise line) at home. The trick is to not get the consumer models.
Latitude: Business (Sales, Marketing etc)
Precision: Developers / enterprise grade.
We had an issue for over a year with Latitudes that would spike in their CPU usage, pinned at 100% and wouldn't release. If we shut the computer down it would be ok for a minute or two when we power it back up then it would go back into the same pattern. I obviously thought it was a virus or inefficient program running in the background. Here is what happened, it was overeating and as a "feature" the CPU would step down it's clock automatically to reduce the heat output. It was just confusing.
The fix for this situation is to run the laptop with the case open, top up so the heat vents upwards. having the top closed creates a blanket effect. I have an Asus G73SW and the same thing happens when I crunch data. This particular model though I have had for a while and it only started doing it after a year or two so it just needs to be de-dusted. Same issue though.
There are some great comments about desktop models though. I personally use desktop and server models for work now. Right now I'm on a Precision T7500 tower that I got on eBay a year ago for like $290. The specs are very similar to an R600 / R700 server which starts at around ~$3k. The mobo supports up to 196GB of ram, has plenty of drive space, onboard raid (get a raid card though for performance), plenty of space for adding dedicated video cards and extra fans, also supports dual procs. Not bad for the price before I upgrade the parts. Altogether I'm at 24GB ram, 16x logical cores and some other goodies for under $1k.
You should also make sure your developers have access to servers. Instead of trying to run everything on their laptops I would advise setting up some dedicated dev / sandbox servers. Perhaps provision a DB server that can be shared and a web server for staging / testing or sandbox purposes.This just helps offload some of the infrastructure load their laptop would normally have to shoulder. In the end some stuff will be run locally and some will be on dev servers (if you have them).
So remember, don't get the shitty lines, go for the Precision. If you have a laptop that is docked, make sure the lid is open and get them a company issue tiny-fan.
Laptop: Dell Precision M6700 or M6800 or whatever the current model is.
Desktop: Dell Precision T7500 or T7600 or the next model up (recommended, tons of room). Oh yeah these workstations are also BIG. It's like the size of a small person and people always chuckle if I have to pick one up and move it around because it looks so absurdly big compared to other models. It's fun for sure.