Looks like after rebates, the 8500GT or 2600PRO are right around $50 (assuming PCIe interface), but not sure how much better they will perform than the X700. I think the 8600GT is supposed to be best-bang-for-the-buck, but generally costs about $75, again after rebates. Newegg has a bunch of 8600GTs for $74.99 AR, some with free shipping.
BTW, Tom's Hardware has a VGA Guide that includes MS Flight Sim as one of the benchmarks. You can compare different cards for different resolutions.