How much does it cost to upgrade a computer?

[C Goto]

I would think about this carefully because there are so many permutations. Some are unfortunately expensive. That is not a recent graphics card for modern games.
You have a low end graphics card which could be good for PhysX though.
You need AT LEAST a GTX 670 or GTX770 graphics card to get any traction in modern games. But you are low on CPU and Ram power.

You actually have to give a budget to get a proper answer; because performance is related to budget.

If you say you have a buget of $200 or 300 pounds then say so. But to get proper performance you have to go beyond that if you want to buy new parts. You could find some good used gear on e-bay (second-hand) or find surplus stick from a major retailer? Ask someone who works there?
If not...
It is assumed you have a 450w-500W Power supply (PSU)? Its important.

But you could get a short-term Cheap/er boost, by going to crucial.com and getting the software to tell you what EXTRA Ram you can buy to improve things for you for the lowest cost improvement of around 40pounds/55USD. You need AT LEAST 4Gb to help your overall system owrk efficiently.

Install CPU-Z to find out exactly what RAM you have as 2Gb is very low for gaming.

I would say the second or third cheapest way to improve your PC running (on a low-end CPU, is to install a fresh copy of windows onto a new Samsung 120Gb SSD, and that should do while you re-install? keep all your other installs/stuff on the existing drive. But you need to be able to follow instructions if you want to install an SSD. Samsung do have free cloning software if you plan to uninstall lots of programs/files you can re-install later.
Second or third option is installing a new graphics card. The cheapest GTX 670 or GTX 770 you can find.
You do not need an SLi bridge to use your GTX650 to run PhysX in one of your mobo-slots (for the games that use PhysX [game special effects]). 125pounds to $150-ish.

But if you want the best rennovation for a proper gaming PC you would need a Motherboard, CPU, RAM and a New Graphics card. thats the reality. A PS4 would be cheaper. But you could buy something pretty decent second hand (depending on where you live); and use your existing PC for light Web browsing?.

Fairly cheap fairly GOOD gaming PC upgrade is this;-
GIGABYTE motherboard intel Z87 LGA1150 ATX standard GA-Z87X-D3H is around 100pounds/130USD: This motherboard is good for Long-term improvements. It does 8x8 crossfire/SLi.
i3 LGA 1150 around 100pounds/135USD (a better `i5` is maybe too expensive [and may need a bigger cooler]).
2 x 2Gb DDR3 Ram around 35pounds $50.
For an extra 150 pounds (225USD) you could then get away with a GTX 760 or GTX 770.
For an extra 300 pounds (450USD?), You could then eventually use two GTX760s to SLi (@ x8x8) and use your 650 to do PhysX only (x4 PCI-e slot). (In NVidia Driver settings)

All in all a tough spot.

I would maybe go for the PS4 if I were you. Or a steam box in a couple of months.