If this is a design time pc/laptop, you're probably looking at 16GB to run them all at once, or selectively then probably 8GB.
2-3GB for the OS
1.5GB for Jdev
1.5-2GB for WLS admin server
2GB for WLS OSB server
0.5-1GB for XE
(roughly 9GB so far...)
Remember that this is enough to install and get them running.
It does not account for a production runtime usecase.
That will depend on your application requirements, design, payload size, desired throughput, component usage, etc