what's the price the 550ti is going to cost, and what would a gtx 750 in comparison cost or even the 750ti? You could do alot worse than a 550ti, I know a friend who streams games on the OEM version of the 550ti, called the GT 545 and he plays fairly modern games well, not all 1080p ultra maxed out with AA on, but I assume most stuff at medium settings, maybe some at high, with AA off at 30fps or more, obviously tweaking here and there can increase FPS.
If a 550ti if all you can afford, then sure go for it, the core 2 quad can still certainly play modern stuff, here's a vid of a guy playing Battlefield 4 multiplayer with a lower clocked core 2 quad and a gtx 650
https://www.youtube.com/watch?v=IRROUneYRcE
"Mostly I have 45 - 70 FPS. Sometimes it lagged because of the recording.
I play the game at 1920x1080 at medium settings.
My PC:
Mainboard: AsRock G31M-GS R2.0
CPU: Intel Core 2 Quad Q8200 @ 2.4Ghz
GPU: Nvidia Geforce GTX 650 (Gainward)
Ram: 2x2Gb Corsair 830 Mhz
OS: Windows 8.1 Pro 64 Bit"
here's a vid of somebody with a 550ti, 4gb ram, and fx 4130 cpu on BF4 multiplayer,
https://www.youtube.com/watch?v=sPGrHSjCzF0
I just used BF4 multiplayer as a standard test of a modern game fairly demanding since it's online, anything lesser than that and your fps and settings could be better, it obviously just depends what you're wanting to play.
but in short, yes, nearly any game ever made will be playable, might be on low or medium settings, but you'll be able to play them.
but if the guy earlier was able to play earlier in multiplayer with only 4gb ram, a low clocked 2.4ghz core 2 quad, and gtx 650, you should be more than fine playing games.
I'm not talking down on anybody here, but people should realize that people in the real world have such a thing as a budget, and maybe they can't afford the newest r9 or gtx 900 series gpu, maybe they're not super enthusiast and are perfectly fine turning settings down and playing more casually. Depends on the person. Not everybody has or cares about spending more money on a 4k monitor, many out there aren't even on 1080p I've noticed, especially in other countries around the world, nothing wrong with that.
OP, making sure you have at least 4gb ram (more would be better) is important as well.
hope I helped some, you should be fine with the 550ti as evidenced by the gameplay videos, just make sure you have at least 4gb ram or more and you're good to go.