Uh, GUYS, it's not only possible, there was a few guys on this very forum who made them (one guy AMD & one guy an nV rig), they came here for help in the build starting with a similar question.
But they were NOT gaming, it was for GP-GPU work, where having the 8 cores was a benefit for number crunching.
Anywhoo, for gaming it's of little use except for multi-monitor situations, but would rarely offer much of a performance benefit over 4 single GPU cards unless you get the models with the 4DVI support for that specific reason.
As for Xfire, the drivers would be a pain to control. Under XP your max support would be 6 cards with a hella-complicated AFR+SFR setup since SFR is limited to 3 frame support, but under Vista you could support 8 GPUs in Xfire, but it would be such a god awful mess to get it to work I doubt you'd see much benefit after 6 GPUS, and likely not worth it after 4, and all you would be adding is complications and CPU overhead.
I can't find the old X2 thread (search function is less than ideal here nowdayas), but I was able to find the older thread from March for the guy putting the 4 GX2 into one rig for his GPGPU rig, (helps when there's more posts on it);
http://www.tomshardware.com/forum/249450-33-four-9800gx2-cards-work
That type of situation is the only good reason to do it, for gaming any more than two of those type of cards is a little useless.