Quote: "But I have a 650 ti, so surely it should at least run at the 40FPS you're getting with your 580?"
Just because the number is bigger doesn't necessarily make it more powerful. The first digit represents the family of the card, yours is part of the 600 family, mine of the 500 family. The family defines compatibility. For example, the 300 series doesn't support shader model 4.0, but the 400 series does. The second digit denotes the power of the card. I haven't been able to find specific information on that, other than the following wikipedia page:
So the GTX 650 would be classified as a "business graphics card".
I'm not sure if this is correct, but the x90s appear to always be just two x80s cards hooked up in parallel.
Comparing your card to mine, you'll see that the GTX 580 outperforms the GTX 650 by quite an amount. I also see that the price of the GTX 650 is ludicrous compared to what it performs:
The GTX 680 is very cheap on this chart, and performs very well.
Hope that clears up a few things.
TheComet