To be honest for the same cost as these card you can land a much more powerful card. $50-75 can purchase a 8800/9800gt, 9800gtx, gts 250, 4850/70, or a 8800gtx. The 4670 is more or less old news while the GT240 is over priced and the gt430 is even weaker than the gt240. The 5670 isn't bad if you can get one for just the right price.
Sorry, I do not have FSX so I couldn't tell you. There's a graphics card hierarchy chart at the end of the monthly Best Graphics Card for the Money articles; look there for comparisons. The HD4670 is close, but weaker. The HD5570 may be similar, depending on the memory type used (GDDR3 vs. GDDR5).