For a budget build the two I mentioned above are built for AH3 in mind. If your budget allows,
mikev just built himself a sweet rig for about $2000 in order it to serve him for several years.
If you don't want to build it yourself, look at the components of those and try to find a matching one readily built. If you want to build it yourself, do as the others and throw a list here for evaluation.
Without knowing where you're from, it's quite impossible to recommend you where to buy from. People here talk about Amazon, Tigerdirect, BestBuy, Newegg. I wouldn't buy from any of them, simply because they might not even ship to my home and if they did, the taxes and shipping costs would double the prices.
By the way, what do you mean by a
complete system? Does that include peripherals like monitor, mouse, keyboard, stick, throttle, pedals... Or just the computer?