Stick with the Crossfired cards until you need an upgrade, and twin HD5770s' will not really give enough of a boost to warrant the expense, they are only a little faster than dual HD4850s'.
I think you are confusing MB with memory bit size.
The HD4850 uses a 256 bit memory connection while the HD5770 uses a smaller 128 bit interface.
While the HD4850 sounds far faster (double the bits), there is little to choose between them because the HD5770 uses GDDR5 memory-which is twice as fast.
More information on your specs is needed, though, fast cards need a fast CPU to keep them fed with data and a small monitor does not need too much graphical power to keep the frames flying.