Logic handles memory like most programs on a MAC which is usually thru MEMORY Allocation. But Consider your soft synth as not just a plugin - but as a separate program that is running at the same time as LOGIC - which is essentially what it is..
Let's take an example - on a 256mg of RAM machine
you launch your MAC OS9.X and it takes 50mg
you launch LOGIC and it snags 60mg
each track you load up takes about 8mg and you have about 10 tracks which translates into 80 mg of ram
then you launch your soft synth which can take anywhere from 30 to 80mg of RAM
which if you total what we have here it can be anywhere from 220 - 270mg of RAM
not to mention if you have any plugins that you have assigned to the tracks -

so as you can see MORE ram is needed to help your system performance, especially when soft synths are being used....