my AA is already turned on in FSX
No it isn't ... if it was then you wouldn't see any jagged edges.
look at the fry's ad in the paper every day and when you see a graphics card at a low price, jump for it.
I would look at a place like Newegg. I don't know what your budget is, but you can buy a fantastic video card for under $100 nowadays ...
Mine's 4 years old
My Dell (see my signature) is also four years old. But she will run with the best of you as through the years I did do some hardware modifications on her, like fitting a Gainward 7800GS+, a Raptor 10.000 RPM drive and 2GB RAM. Let alone the countless hours I spent with Nick Needham and Michael Greenblatt learning how to balance my machine for maximum performance.
I sometimes service computers for my friends and I am amazed how poor they run! Even the newer ones run like crap. Their drives are a mess (full of junk and never defragmented) and if you open op the case you can't see trough the dust. Clean the inside of your case with a vacuum cleaner every now and then (BE VERY CAREFULL NOT TO DAMAGE ANYTHING!!) In short: many computers are not looked after ...
I often compare computers to cars; cars need servicing every now and then. If you want a machine that will run Flight Simulator properly, then compare that machine to a Formula 1 racing car. Setting up a Formula 1 car is a very delicate job, that requires a lot of knowledge! So is setting up your machine for Flight Simulator.
