RAC on milkyway for a 7970 should be around 300+K its around 35-40 sec per unit depending on how many you do at once and what your overclock is, you get 160 points per WU no matter how fast you do them.
A 5870 takes around 20 seconds longer so RAC is around 200K, same caveats apply.
To get that sort of PPD on Folding@Home you need a quad dodecca core MagnyCours rig, then you will get around 400K to 600K depending how fast the cpus are.
And yes its gonna get hot in there, and as the ambient temperatures keep rising so will the rig temps unless you can cool the ambients down or exhaust the hot air from the rig somewhere. Welcome to the world of Distributed computing, great in winter, but a PITA in summer!
Water cooling only helps so much, you still have to get rid of the heat somewhere and unless you can keep the ambients down even the best watercooled rig will just keep getting hotter and hotter.