First some good news: The CPU is still a good one, as are the RAM and motherboard, you still have a decent, if older core system, you could upgrade but the performance gains will be relatively small compared with a graphics upgrade.
By current standards the GTX550Ti is quite weak, though:
http://www.tomshardware.co.uk/gaming-graphics-card-review,review-32899-7.html This chart is now getting outdated, Toms haven't updated in a while so it's missing several new developments.
If you're sticking with the current display look around the GTX960/R9280/R9380 class (last one's not on the chart but it's essentially a slightly updated R9 280) they're all streets ahead of the '550Ti and not too expensive.
If you want more eye candy or plan on upping the monitor to 1080 res go higher; the GTX970 or R9 290/R9 290X (or their updates the R9 390/390X) are all massively powerful parts ideally suited to high settings gaming on 1080 or even 1440 displays.
Whatever upgrade you decide on, don't go cheap on the power supply, it's the bedrock the system is built on and cheap ones can, and have, killed systems when they fail.
For a single graphics card, non or lightly overclocked system 650 Watts will be enough, if you're planning on heavy CPU/graphics overclocking go 700Watts.
A Few Good Names: Seasonic, Silverstone, XFX(Pro Series), Antec, Superflower, Corsair (not the value 'CX' and 'VS' series), FSP (Aurium Series), Rosewill ( Capstone Series).
Prices and availability vary obviously while modular or semi modular parts don't fill the case with unwanted or unused cables which can block airflow and be a real pain to tuck away in a great many mid tower cases.
And no, a GTX980Ti/850Watt combo is a huge waste of money unless you do a HUGE upgrade to the core system-CPU,MB,RAM and get at least a 1440 display to go with it.