Planespotterx your problem is three fold....
1) You have a Duron, which is the toned down low end version of the Athlon. Just like Celeron vs. Pentium. And granted, Durons are very good for their price... but they just don't pump out the same horsepower as their big brothers.
2) You have the MX
version of a GeForce card. The MX
version of ANY GeForce card equals crap.
3) You don't have enough RAM. In reality 128mb is the minimum for FS2004, but you should really have atleast 256mb.
The freezing problem might have something to do with drivers. Make sure that you have the newest drivers for all the components in your system... video, audio, etc...
As for the frame rates... sorry but the only thing you can do is keep turning down those graphics settings and resolution in FS2004.
Just to compare this is my system.
GeForce2 Ti 64DDR
I run FS2004 at 1280x1024 with 100% AI
and all scenery detail between medium and high, all aircraft detail maxed out and I get about 7-10fps in dense scenery and around 15-20fps otherwise... not great but good enough.