Well, I can't honestly tell you much about Hybrid CFX, but I think it would require a second/third Radeon HD 42xx card. I'm not sure, though, as I've never seen an actual discreet 42xx card for sale.
It really wouldn't matter, though, as a single 5850 would probably smoke three 4290s. I'd even take a 5830 over them.
If one 4290 can only pull 16fps in STALKER: Call of Pripyat at 1280x720 on low detail with no AA, then I'd say three of them wouldn't be much better. You'll have to have someone else confirm whether or not you can put a GPU from a different series with the integrated graphics, though, as I'm really not sure. I'd go ahead and get a 5830, 5850, or whatever it is you wanted...
don't even worry about any kinda of crossfire with that iGP, if it's even possible, if anything if you did crossfire any moderate powerful card to the IGP you would most likely lose fps over the one card by itself