Sun 5.8/Oracle 9.2

So for eg. If my Physical RAM is 1GB and i have 4 Instances each with 400MB and if my SWAP AREA is 1GB....what happens ?...

Will it stop loading from 3rd instance since 800 MB already used for Loading the First 2 Instances ?...
Iam new to DBA and iam trying to figure out the MAth here ...
Thanks