I would agree, a 5850 would be an excellent choice for you.
TBH though, FSX is extremely CPU dependent.
You may want to check if your Dell BIOS allows you to overclock your QX6700.
If it does, spending a little of the left over budget on a good CPU cooler and pushing it to its limits will be quite beneficial.
You may want to brush up on your history a little...
A DDR3 4650 will perform very closely to a 7950GX2 and anything faster will outperform it.
Besides being several generations out of date, the 7950GX2 really is not that powerful of a GPU...
Without having the time to link you piles of reviews comparing cards through the several generations, I will instead leave you this link.
http://www.tomshardware.com/reviews/gaming-graphics-card-geforce-gtx-480,2598-6.html
While it may not always be 100% spot on, in this case it follows the reviews I have seen/remember quite closly.
As I remember it, the 7950GX2 is, on average, 15-20% slower than a 8800GTS 640.
Jumping forward a bit, the 3850 performs pretty darn close to a 8800GTS 640, maybe 5-10% slower.
A DDR3 4650 is around 10-15% slower again than the 3850.
Looking at it like this, it really is quite close in performance, probably just a little slower.