You will not be needing more than 2 gigs of VRAM running at 1366x768 resolution. Almost any game can be maxed out at that small resolution. I would honestly recommend the 750ti, try to save a little more, but if the 730 will be fine as well at that resolution.
VRAM usage tier: Resolution takes up the most, than texture quality, than Anti-aliasing (AA can use quite a bit if you really are cranking it), but at that resolution you will have no issues.
At that resolution though older cards would really shine. The GTX 480 for instance would demolish any game at that res, or even the 580. You could go through ebay for those.