Well the
750ti doesn't require a power connector and is probably the best card you can get for low profile cards, the
r7 260x uses more power and requires a connector. it really depends on what your comfortable with, either one really you'll want a better power supply to go with it, then at least if you do upgrade fully you'll have a power supply and a semi decent gpu to put into your new build. all you'd have to save for then is a mobo, cpu ram and the other smaller peripherals like a hdd/dvdrom/kb/mouse etc etc
you can pick up motherboard, cpu and ram bundles quite easily these days usually priced quite well too. if you plan to upgrade fully there is nothing stopping you doing it slowly, using the parts you can afford now in your current rig, then swap them over when you can afford the rest. shop around though, its quite easy to figure out whats best for your price range (most places are very layman.)
because your doing 1080p you'll probably want a 2gb card, for a powersupply, like treelife said above try aim for one thats 80+ bronze perhaps something like
This, either one of those cards and the PSU should bring you to about $200. i think you'll see quite a reasonable performance increase over your 7750. (
example 1 )(
example 2) the r7 260 is a bit cheaper than the 750ti but, what you save on the card you make up in power consumption. you could also check out the
r9 series although they are a bit more expensive bringing you right to your budget cap.
thing to note is that, ddr4 ram is supposedly being released this year, so, if you hold off on upgrading the mobo/cpu/ram until then you should be able to save quite a bit on ddr3 ram. the price should come down on it. which means the bundles will drop in price too.