4 gigs of memory is more than sufficient for fsx.
FSX is more CPU hungry than GPU, so best improvement would probably be o try ot overclock the CPU above 3.0GHz. Your success with this wil dependon the individual chip, whether the mobo will let you and whether you can still keep it cool.
The 9800 GT with 1 gig RAM (didn't know that thre was one with more than 1/2 gig) wold help little, but it's the CPU that is going to limit you.
Even this would only make your system marginal for FSX at best, so it's probably time to be saving for a new system or stick with FS9 for a bit longer.
Q9550 @ 3.78 GHz with Gainward GTX570 1.25 Gig DRAM
4 Gig DDR2 RAM - Windows 7 64 Bit
Resolution 1680 X 1050 X32