Now i am a bit baffled by this post.
You say a 5870 and it responce is sluggish ?!?!
I am running also a 5870 and believe it or not while running any project still can do games/work/surf except when i set my games to ultra high settings.
For normal operation i have in no way any experience of a sluggish screen.
Hell even on my other old ati 4770 i can surf, wordprocess and do whatever and not even see this. Ofcourse when i start gaming on that machine i see a result.
Now to find the cause of the sluggish reaction there must be ofcourse a reason.
One reason could be that you run the card in a slower system.
For instance i was running the 5870 on a 2,4 Ghz Q6600 and when i ran 3dmark06 benchmark only got 14835 points, so i overclocked the cpu to 3,2 Ghz and the immediate result was that i got 17821 score in the same benchmark

But i think you must check for how many pci-e slots your system has if you have more then 1 slot, which slot is your video card in ?.
And second does your system has other addin cards in the normal pci slots.
If so some of these other card is maybe fighting for the same irq in your system this can be sometimes solved by changing the slot where the videocard is put in or the pci card ( mostly sound/network cards)
If you want to check if an irq is conflicting you can check for instance with hwinfo/everest/speccy or other system info programs.

I quit folding for awhile so I also stopped coming to the forums here..hence my reply just now. But now that the weather has been a bit colder here in Indiana..probably start crunching.

For my setup..you could of just looked under my name..and no I don't have Crossfire its just 1 5870 and a overclocked AMD X6 1090T to 4GHZ.

I ran some other folding projects (can't remember exactly) but my computer wasn't laggy with some of those. So its not my computer..

As far as gaming while folding..lol yea right this project takes up almost all of my GPU resources to game while this is happening would be nothing but a FPS lag fest.

For the PCI-E slot..its in the correct slot..a 16x pci-e one. I can still surf the net and stuff but there is a noticeable lag in it.