I had a 4870 and a 4870x2 before I got my 3 5870s. I can honestly say that the biggest advantage for me seemed to be higher min FPS, and just a nice little boost in avg FPS.
Now to answer your real question, all 3 cards will run fine in trifire, as they are all at the same clock rate and memory (it will down clock the proc/memory speed, and amt of VRAM to the lowest amount) so assuming you have a 1GB 4870, you won't lose anything with the x2 and 4870 crossfired.
Resolution is what's really important though, as I game at 2560*1600, so for me it was worth it to pay the extra to help out with the min frames with AA enabled, but the lower the resolution, the more your performance will be minimal (and maybe adding the extra overhead from the second card will actually give you less performance) like at 1680*1050 maybe? Not sure, but the lower you go, the less GPU dependant it will be, and the bigger waste of money the second card will be.
What resolution are you playing at? Oh, and btw, the reason I got a 4870 and not another x2 for my second card is that it performance was better accross the board at the time with trifire than quadfire. (as stated above, drivers might have improved perfomance dramatically since then though) can't really comment on that though, as I believe 9.8 or 9.9 were the last I used with my 4870s
BTW, while I opened the thread, what's the real value of a 4870x2 now? I see the prices on sites like ebay and all but without a solid source like newegg or tigerdirect selling the card, I have no idea.
Wow. I didn't know that they used THAT much power. Well, upgrading the PSU isn't too much of a problem however, I don't think I'd want my pc drawing that much power. Would it be a better solution to just sell my 4870 and get a 5870?