Ok here is the deal... I am playing FS 2002 for a while now but i cannot add any scenery cause my fsim will then be too slow to play anymore. All i want is to be able to add some airport scenery.

Here are my specs :
AMD 1.3 GHZ processor
384 MB of memory DDR-SDRAM
GF 2 128 MB Graphics card
120 GB 7200 RPM Maxtor HDD

Is it possible to get better playability when i opgrade to lets say 640 MB of RAM  ???

Congo
To answer your dual channel mainboard question:
Dual channel RAM in Athlon systems do not increase performance over CL2 single channel ram.
THG tests show that clearly.
In THG, search "dual channel" and several tests will show.  Not a material difference dual to single if single is CL2.  The issue is one of cost also.  Is dual RAM cheaper than CL2 single?
It's a frontside bus issue.
I plan to test a DIMM of PC2100, CL2, 512MB.  I'll remove both 256 no name brand stuff and run with the single stick and see if there is an improvement.  Should be an improvement as latency is 25 percent less CL2.