the only difference is the capacity. 1 is becoming a little too much while 2 is just right, 3 is typical of higher end cards and tends to be great a higher resolutions while 4 is pretty much only used in 4k or triple screens.
for one 1080p tv the gtx 760 is a better choice as you will never use over 2gb of vram and the 760 is faster than the 270x