Yeah, Flight Sim is a spacehog, for sure. The RAM you added doesn't have a thing to do with drive space ... its in use only while an app actually is running. Your hard drive ... what Defrag/Disc Useage deals with, is entirely a different critter. More RAM is always a good idea, there's never enough, no matter how much you've got or how much your machine will accommodate. 4 Gigs of PC 1600 RAMBUS is prolly close to enough, but that's all this machine will take, so if its not, I'm screwed
Couple things you might consider - offload Flightsim and play it from disk when you wish, as opposed to having it resident on your drive. Just cleaning out any saved-but-no-longer-wanted game sessions might help a bit too. External hard drives are relatively cheap, and no hassle at all to install, and any "Permanent Personal Files" ... media, spreadsheets not in progress, photos, letters and that sorta stuff can always - and preferably - be offloaded either to writable media or to an external drive. There's prolly a buncha stuff on your drive you haven't accessed for a long time and likely never will, or at most, will access only infrequently. You'd be amazed how much junk winds up on a drive. Do some digital housekeeping, and you might be surprised at the improvement you see.