I'd get a Athlon 64 X2 3800+ and overclock it to 2.6Ghz like most people are
doing with that core. Most games, MS Flight Sim Included, can not fully
take advantage of dual cores. However, MS FS is extremely CPU intensive.
Having dual cores can help if you designate one of the cores solely for the
game while the other core handles windows etc... In addition, and this is
where dual cores can really shine for MS FS, there are so many third party
add ons that run simultaneously and if you had dual cores, you could run
these programs on the other core. Things like traffic programs, video
capture programs to save videos of your flights, flight planning progs,
etc...
MS FS is one of the very few games (or sim if you prefer) that I play and it
is the only game that really pushes my system (which is pretty good for
flight sim).
As far as straight up performance the fx57 will beat the x2 4800. But the
x2 can likely be overclocked with the right components. So if you compared
a single core running at 2.8 vs a dual core running at 2.6 the single core
would only very marginally be better. In fact you probably would be hard
pressed to tell a difference in Flight Sim if that was the Only program
running. But like I said, having that extra core, helps free up some cpu
resources. The kicker is that there are going to be more and more apps that
come out that take advantage of the dual cores.
Roy