Not sure why that is. I have a HD 4870 running in a Core 2 Duo processor machine. I run work full time on both cores of the CPU and it has virtually no effect on the production of the GPU.
Well, if the 5970 is anything to go by you'll also need to worry about the size of your case. That thing is 32cm long (12.5") and weighs over 1kg!The HD5990 should be awesome but unless you run it on an I7 you may spend a lot of time waiting for work. Milkyway still limits you to only 6 WUs per CPU core.