I don't think the 550 Ti is nearly as awful as everyone says, I actually like it quite a lot. Plays Skyrim at ultra @ 1600x900. But at the same time I would agree you should spend a little less on a CPU and more on the GPU, I have a G860 that is just fine for the 550 Ti. I'm sure you'd see much better performance with say a 660 and and i3 Ivy.
If you realy need the i5 processor over something with less powerful then i don't see why the 550ti is a bad option. You can play the games you choose at the resolution that you want to play fairly easily, yes given the 660 geforce card is much more powerful and would be an overall better option for gaming, if you are mainly using your system for games and video editing. In the end if you realy feel that your 550ti isnt realy cutting it for you in games then i dont see why you shouldent upgrade since the 660 is a lot better when gaming and the performance increase is defiantly noticeable.
I have been gaming just fine with a wolfalde pentium dual core @2.8 and the PNY 550 ti, but I'd like to try PS2/Wii emulation which requires a i5 quad core to run at 100%. Besides the price difference between the i3 and i5 is just $50.
and this way I future-proof myself if I feel like upgrading next year to a 1900x1080 monitor and a GTX 660 ti.