I used PC Probe to determine physical RAM availability for three MS flight sims.  512MB RAM installed:

FS 2004  181KB remaining; 63 percent RAM used.
CFS3         76KB remaining; 83 percent used.
CFS2       247KB remaining; 52 percent used.

CFS3 settings are conservative with T&L off, Shadows off etc.  Clearly CFS3 needs more than 512MB RAM to function well.

FS 2004 does well with 512MB RAM.

CFS2 RAM demands are modest and that is why it runs so well on most computers.