Processor upgrade makes no sense, performance benefits will be almost negligible. As somebody else said, there is nothing that you can do with a 2.8 ghz cpu that you can't do with a 2.4 ghz. You're talking shaving a few seconds off a task that takes dozens of minutes to complete.
Upgrading RAM makes not much sense either, extra ram will not speed up your system unless you're using it. Take a look at your current usage in the activity monitor. If you have no page outs then you have never exceeded the usage of your current system. I've got 8gb in my 2009 mbp and I've never had a page out. Even when I'm playing games (the few games I can play on this machine...) I usually have 3-4 gb free. It's true that you'll use more ram on a retina system using retina apps/resolutions. but it's not going to be that much more. Most gaming pcs use 8 gb of ram, it's the standard. 16 gb is overkill unless you're running virtual machines.
I would say that the ssd is the only upgrade worth getting, counter to what everybody else here is saying. Being able to put all your stuff on the internal disk and then backing up externally is so convenient, much more so than having to split your content to an external drive. Yes the ssd is about the only thing you can upgrade after purchase but the OWC drives are more expensive than apples (admittedly you get to keep your original drive) and they only go up to 480 gb. The 512 gb upgrade is a solid choice in my opinion. The 768gb upgrade is overpriced, it costs more than double the 512 even though it only offers double the storage. But if you have that much data maybe it's worth it for the convenience.
Personally I'll be getting a base level haswell rMBP with at least 512 gb ssd, so long as they don't screw over the gpu. Here's hoping for a GTX 765m...
