Yep, so your powersupply is only capable of putting out 16 amps on the 12V rail. That is pretty low. Now depending on how many other items you have hooked up in your case, you probably do not have enough amps left over to "safely" run a newer video card.
If you only have one hard drive, one CD/DVD Rom, NO USB devices, no sound card, etc... you MAY, just may, be able to get by with it, but I doubt it. Even if you are just barely able to run the new video card, you may be helping to cause it to overheat. Why? As the video card tries to draw the amps it needs from you weak psu, there is just not enough "juice" to give it, so it causes things to go horribly wrong! lol
My recommendation would be to purchase a new psu. Get one that has an amp rating in the high 20's, like 28, or higer in the 30's if possible. The wattage is not quite so critical like the amps, but since you are going to purchase a new psu anyway, get one that will last you for a few years. So, consider the 650 watt psu's as a minimum. But again, the amps are the MOST important. So, if you find a 500 Watt psu that can deliver 30+ amps on the 12V rail, that should do you for the future unless you plan on trying to run 2 video cards in SLI. In that case - get a 700 Watt psu as a minimum and be sure the amps are close to if not over 30 amps.
Note that with multi-rail psu's, you can not simply add the amps of the various 12V rails together to get the total amps. It does not work that way.