550 is pretty low for a 4890 it sounds like you've already pretty much isolated it to the psu so if you've already checked updating your drivers and such thats probably the problem, also you might want to check your temps to make sure nothings overheating
If you do a search at google for HD 4890 reviews you can read the technical reviews for yourself. Most of the reviews were about game performance at maximum settings. The HD 4890 is a high end video card intended for extremely serious gamers. If you are a hardcore gamer into serious overclocking, then you might want to consider upgrading your power supply. One of the reviews mentioned this issue when using a game's maximum settings.
As serpjo already mentioned updated drivers and a high temperature could also be problems. In fact, I would suggest checking for updated drivers first because you stated you pc hangs not only when you try to play games but also when you try to do something else.