What would I get out of spending more cash on 512 M more DDR under Win-XP to get to 1024 ram>?(P4-2.4/512RAM/128V-RAM)

Probably a little bit smoother experience. Less hesitation when going into areas where new scenery gets loaded up. Certainly less hesitation when switching views. Like going from looking straight forward out your cockpit to looking out the right window. If your using PanelAsTexture=0 in your FS9.cfg right now, you could get rid of that line, as you would then have enough ram to keep your Panel loaded up in ram rather than having it cached.