Kernel trap has an article on How to Use RAM as Swap. Basically you'd setting so that when you run out of physical memory instead of going to the hard disk for storage space, you move that into ... uhmm.... physical memory. Hmm.... Well, read the article and decide for yourself :)
This is just silly... You would be better suited to having the memory available as free memory AND having a hard disk based swap partition/file.
It's just inefficient to move little used data into yet more memory for no reason... And what if you need to dump the whole system memory in the event of a serious crash for debugging?
It's better to have let the VM choose what to do and give it what it's expecting as swap: a really slow but permanent storage system with more than the systems volatile memory of storage.
That way it can map memory back and forth without problems - and without wasting precious physical memory for a negligible performance increase when accessing little used data.