The Crossfire thing is true, but requires the cards to be of the same family.
for example, you can run in a dual card setup any of these cards, Radeon 4890, 4870, 4850, 4830 with each other, even a 4850x2 or 4870x2 with any one of the cards above for a 3 way Crossfire setup. (you can mix clock speeds and memory sizes too from what I've been told, Crossfire isn't as rigid with standards as SLI is, but SLI seems to usually be the better performer from what I've seen, plus drivers are usually more polished for SLI as well, and updated more frequently too)
but its recommended to run the same cards, so 2x 4850's, or a 4850 and 4850x2 for example.
but you can't run say a 4670 and a 4830 together, their not of the same "family" of chips, if it were a 4650 and 4670, yes. as to the 4770, as far as I know, they're all by them selves so far, since their the only 40nm GPU's out on the market from ATI.
my Core i7 setup, if I ever get around to finishing it LOL, is supposed to be running a 4870 and 4830 in crossfire (have all the parts, just need to finish assembling it and install windows). although I'm only using one display now or will be, my 24" LCD, running 1920x1080, I plan on getting a smaller 2nd 15"/17" or so display sometime late in the year, something running 1440x900 or 1366x768 or something in that range. so that when I'm working in my 3D apps I can still keep an eye on other things in the smaller display (as it is the software I use will allow me to split the screen of a large display into 2 windows so I can have a more effective workspace layout, I think one of the more recent Photoshops allow something similar). but I have to research some more and find out if Crossfire will allow me to run multiple screens or not, I only need one for games, but whether it will allow me to run games on one display in crossfire, and allow me to use a 2nd display for desktop and any other apps? lord knows the i7 will be able to handle it, just have to worry about the drivers for the cards, thats the limiting factors in my situation.
was going to comment on your reply about the PCIE x1 slots, but I scrolled down further and see you found what I was talking about.
if its a x16 "Physical" slot with x1, x4, x8 or so bandwidth (commonly referred to "Electrical") then yeah, a 2nd card would fit and run/work, but it needs to be a physical x16 slot to fit and run. although its possible to "mod" a x1 slot to run a x16 card (Karmakiller or someone, 2Monsters maybe) had a link from another site where they modded an x1 slot or x4 slot to run a full length card, for Folding@Home. but needless to say it involves dremeling and filing and potentially destroying your nice new motherboard to work LOL. there are other ways, like using a x1 to x16 adapter card, which is intended for low profile cards, but the adapter cards cost another $30-50+ and all they are is a Riser card.