3rd party RAM from Crucial, OWC and others is already available for sale.
http://www.crucial.com/store/listparts.aspx?model=Mac%20Pro%20%28Late%202013%29
http://forums.macrumors.com/showthread.php?t=1660838
I think it is almost a toss-up on the 8-core or save and go for 6-core.
This is a quote from Marco:
You can also see why I don’t recommend the 12-core model to anyone except those whose software will definitely make very good use of all of its cores, at least most of the time — because for any other conditions, it’ll be slower than the others.
On paper, and these Geekbench results from the 6- and 8-core support this, it looks like you really can’t go wrong with any of the CPU options below the 12-core — the deciding factors should only be cost and how much parallelism you can take advantage of.
It looks like you’re paying a lot for slower clock speeds as the cores increase, but that’s not the entire story. Those weird Turbo Boost numbers, which are easy to pull from here and here, are worth understanding before choosing a modern Intel processor.
http://www.marco.org/2013/11/26/new-mac-pro-cpus
Mac Pro 2013 6-core Geekbench Scores
http://www.marco.org/2013/11/05/geekbench-mac-pro-hexcore
(2013) Mac Pro review
2013 Mac Pro review: small, fast and in a league of its own (engadget)
Tested: New Mac Pro is the speedster we've been waiting for (finally)
You probably are not locked into a graphic device.
Apple hasn't officially stated GPUs are upgradable, but the general concensus based on the design is that you'll likely be able to change the GPU. But agreed that's not for certain.
http://www.besttechie.com/2013/12/25/can-you-upgrade-the-parts-of-a-new-mac-pro/
http://www.macworld.com/article/2082515/mac-pro-late-2013-review-apples-new-mac- pro-really-is-for-pros.html?page=2
