AA is alcoholics anonymous....j/k

It's actually Anti-Aliasing, basically where the video card renders each frame more than once and combines the renders together to get a smoother final frame that is displayed. (it actually has to do with signal processing, but that could take a while to explain, that was just the short and easy version)
You won't have to worry about a PSU, even if it is only 250W they power consumption on you system doesn't sound like too much and the 5550 won't cause any problems. I have a 325W system and I have 3 hard drives, an x1600xt, sound card, modem, wireless card, several case fans, cool case lights, and a AMD 4000+ SD core, with no problems.
I hope you end up getting a new computer, that’s always exciting. In the mean time the 5550 should work just fine, however, I don’t know how it will work for Legacy.