[CentOS] RAM for cluster net boot?

I'm looking at building a small cluster of disk-less 1 or 2U servers and 
will probably use CentOS 5.

Since these machines will not have any hard drives, what would be the 
minimum amount of RAM I'd need?   Also, if using Rocks or something 
similar, will that help cluster the RAM together so 4 servers x 4 GB RAM 
each = 16 GB available?

Some applications might be CPU intensive, others might be RAM intensive, 
so I need to play that balance, too.

Thanks.

Scott