Minimum spec for a nVidia 560 is 450W. That *could* allow for a little wiggle room with a 430W PSU, depending upon the rest of the system.
Everything inside wants some Watts, even some thing not so obvious. I have four hard drives and can hook up a couple of USB devices at times. These all want some of what my PSU provides. If your system has one HD and a couple of sticks of memory, then your PSU could provide the wiggle room to go to something like a 560.
A nVidia 550ti only wants a 400W PSU and that will give you a much better card than the old ATi, but a little less ummph than the 560.
My original FSX computer that I bought six years ago had an ATi X1900XT and I saw an improvement when I later put in a nVidia 9800GTX+ and more improvement when I went to my nVidia 560.
So you have a couple of options. The 550ti is a safe upgrade for around $120 and won't require any other system changes. The 560 *could* work in your system, but that's taking a chance. Or third choice would be to grab a sale on the U.S. Black Friday where you could get the 560 and a PSU with a higher wattage rating. Staying within $200 should be possible.
And I asked about the computer case because sizes/lengths of video cards can be a problem in certain situations. The ATi I have is similar in size to the nVidia cards, so I wouldn't imagine any problems there. But it might be worthwhile to take the side off (with the power off! and cord unplugged) and use a ruler to see how much room there is. If your computer is fairly recent, last couple of years, then I'd be making sure that the PSU connectors for a new video card are available. Generally, unless you were trying to put today's behemoth video card into a six year old system (thinking of my first built FSX system) then power connectors shouldn't pose a problem.
