Most graphic cards available today would handle flight simulator on high settings, but you would probably get the best bang for your buck with a ATI 4650($50) or a ATI 4670 ($65) Anything more powerful would likely be bottlenecked too much by your older processor.
If you want to spend less, an ATI 4350($35) or 4550 would also provide good performance in that game, but wouldn't perform as well on new games.
www.newegg.com is the best place to buy graphic cards--its prices will be half what you find in stores.
Before you purchase you need to look and see if your computer has a PCI express x 16 slot or the older AGP graphics slot. This will make a big difference in what you can get.