charrua -

What Operating system are you running? It makes a difference as to whether your system will see all 4 gb or not.

To the PC-6400 (or ANY other memory buyers). Make sure that your system POSTS and sets your memory clocked to 800mhz or above - otherwise your PC-6400 is just not working fast enough. by default, many motherboard manufactures clock memory based upon FSB and the CPU. By default it may set to PC-5300 or anything else. Check your BIOS and your MB docs.

RealFlight and many other applications - performance is not specific to CPU, but instead mem SPEED (not just total mem), the GPU and for applicatiosn with a lot of disk I/O - the harddrive performance. You can have 16Gb of tyhe fastes memory around, but if its running at 400 or 600mhz, what's the point?