How well will X4500HD handle an older program like Flight Simulator X?
FSX remains the most CPU bound gaming title available today. It demands as much CPU horsepower as you can afford to throw at it. FSX frame rate is ~ 85%dependent upon CPU Cores and overclocking, so an i7 @ 4.0 Ghz yields responsive frame rates.
Although FSX runs best on mainstream nVidia GPU's, it also runs best on a single GPU, rather than SLI / CF / X2 GPU's, since multiple GPU's divert valuable CPU interrupts, which has the effect of actually reducing frame rates.
See Tom's Gaming Graphics Charts Q3/2008 - http://www.tomshardware.com/charts/gaming-graphics-charts-q3-2008/benchmarks,30.html
I've been a Flight Sim enthusiast since 4.0, so I've built all my PC's over the years as dedicated FSX rigs.