Well, there are limitations inside X when combining Xinerama and compositing. I don't know if anyone is even working on fixing those.
Search for other posts from JaXXoN over there, he's spent a lot of time on >2 monitor setups.
* use two DualHead2Go / TripleHead2Go (they're basically DVI splitters)
* use an nvidia card with 4 outputs (I don't know any, IIRC there's only quadro card upwards of 1000 bucks)
* use an eyefinity card with 6 mini-DP ports. Note that you'll either need new DP-capable monitors or four active DP->DVI adapters (~50 each). Also note that eyefinity isn't supported by fglrx yet, but they're working on it.