Yes. I actually have 2 XPS 8300s and after I put 32 GB in one of them as a proof of concept, I split it into 16 GB per machine. They came with 8 GB and 6 GB so it actually worked out to be 20 GB per machine. One is running Win7 Pro so that uses 20 GB and the other is Win7 Home Premium so that shows 20 GB (16 GB Usable) in Win7 properties. I figure at some point I can upgrade Win7 on that machine to get the full 20 GB if needed. Otherwise I didn't have anything else for the RAM to go into so I left it in there for the heck of it.
So your proposed configuration is EXACTLY what I'm running in two machines. BTW, the BIOS on the two machines at time of upgrade was A06 and A03. I have since upgraded the A03 to A06. So the extra RAM works fine even on earlier BIOSs.