I would swap the two 550ti's for 1 570 or whatever is a similar value.
no microstuttering, more vram as you only have 1Gb of vram and at 1080p that will cause you problems, 2Gb is better, cooling will be easier, can you afford a 670? probably not.
At the absolute best the sli 550ti's would do twice as well as this comparison suggests. in which case loses many times to a 570, however in reality they will only scale to about +80% max http://www.anandtech.com/bench/Product/541?vs=518
I would however change the 3770K for a 3570K, and put that money into the gpu, could this get you to a 670? if it does you are set for more years than the 550ti's
64bit for sure.