Disk v. Mem

[ashen]

erh, at the risk of getting bounced out of
here for asking a fourth-grade question... ;) 

what are the advantages of swapping out objects
to disk as opposed to keeping them in memory?

let me stab in the dark:
do you really need to save that much memory? 
do you need it for something else? 
are objects so large that 40,000 of them fill mem quickly?
does too full a mem size inhibit the driver speed?
isn't it more costly in speed to write to disk? 
what kind of savings do you get versus this extra cost?

i'm seriously inquiring here...before i've just let the
whole subject go by without thinking on it too much.

-john