I checked the power draw on the 670 2GB and it's 170w but you need to take into account what power connections the 670 needs (1x6pin and 1x8pin). The length of the card is 275mm so you'll need to bust out a tape measure to check your case. Although the 670 is a PCI-E 3 card it won't be a bottleneck on a PCI-E 2 motherboard.
In addition to the above you have to make sure that a new video card will physically fit in your system. Some of the bigger cards (you mentioned a gtx670) won't fit in many cases. You'll also have to make sure that your system can handle the power and that the pci-e slot has enough space around it for the bigger cards.
Did you build this computer or is it stock? Can you get us the model number so we can help you find if stuff will fit? Also, your budget and what you want to do with GPU will help.
Any card will work, assuming your power supply can handle it. If you have a prebuilt machine, this information might not be easy to come by, sadly. Many larger graphics cards require an auxiliary power cable. The GTS 240 does not require this, so your machine may not have one. It's also possible the power supply simple does not output enough juice to run a bigger card. Check your manual, see if you can find any specs on the power supply, and check inside the case for a GPU power line (it should look like a bundle of wires with a square connector on the end, with 6 pins on it. It may or may not have an extra connector with 2 more pins hanging off it.