From all I can garner about these cards you have to solid cards. If you want VALUE and performance then the (xfx)7770. If money is not an objection then go for the 7850. But I have read many reviews about the 7770 performing well playing Skyrim as well as the other games.
As for the hard drive 5400 rpm, if that is your only Hard drive I would get the 7770 and spend the $40 towards a 7200 hard drive. ---->>> here is a review of 5400 vs 7200
http://www.tomshardware.com/reviews/2tb-hard-drive-constellation,2630.html
The 5400 will do just fine unless you are loading maps, etc..stuff like WOW and MMO's from what I have read.
As far as your concern for a bottleneck I don't see it being caused by the graphics card performance. You have a very good set-up so I would only change 1 thing at a time and if you are upgrading the card to play then this would suffice. You have MORE than enough computer there to do it. You made a good choice.
There is no such things as overkill lol...Take care
I wanted to edit this as I am seeing reviews that say the 7770 would not be a good choice. I am speaking from experience as I run the XFX 7770 card and I am very happy with the quality and performance when on max. Yes there are cards that will perform better on max...but there will always be a card that displays better.
Again good luck.