It depends on your budget of course. You can crossfire at X8 (790GX, 790X), or at X16 (790FX). The difference in bus speeds will only show up at higher resolutions and large video cards. If you are talking two 4890s on a 30" monitor, then you are probably looking at 10FPS, which might hurt in some demanding games.
As far as DDR3 goes, there is no real performance benefit. However, there might be in the future. If you buy 1600Mhz DDR3 now you will probably only run it at 1333, but DDR3 will be around for a while and maybe you'll be able to reuse it at some point.
Also, upgrading to 8GB will be cheaper in the future if needed, as DDR3 prices fall and DDR2 rises.
One last thing. The memory CONTROLLER is on the CPU. It's possible that AMD will make future CPUs without DDR2 support, making them AM3 only and not compatible with AM2+ DDR2 boards.