the 7900gs should handle FSX on high settings alright. the 7900gs should last you a year on mid-high settings for upcoming games as well, but if this is an upgrade to last more then that...i suggest getting a 7900gt or gtx if you can. Im not sure if they offer the gto in the uk but try to find it if you can...evga makes them, and their cheaper then the gtx but with ocing they are the same.
Another thing...im not fimiliar with the company you picked, but im not too fimiliar with the uk in general :P. I did happen to notice an ad for the foxcon7900gs on the same site, and foxcon's are known to be reliable. I would have suggested an xfx, evga, or bfg, but i don't think they supply to the uk.
edit: heres a benchmark i found for the FSX demo and 7900gs...sry i gotta get to writing a research paper so i can't find a benchmark for the retail FSX and the 7900gs for ya rigth now
edit edit: guess i just don't want to write that research paper
. There online store has the bfg 7900gs
and the evga 7900gs
. The foxcon sould be ok but try to go for the bfg or evga if you can.