Not too many things need 4GB of ram. I would surmise, that in about a year or so... Maybe, but unless you are doing some SERIOUS crunching... (CAD work, Rendering, etc...) would you need that much ram. 2GB is fine.
as for the 6600. good choice. it's the starter that has 4MB L2 cache. I personally have the 6300 and have had no issues with it at all. but I don't do enough to need the extra speed. (I don't do extreme OC'ing)
and the GTX... something new will be out in a few months, so unless you want the "best" for this month, I would suggest getting the GTS and calling it a day. (Which, BTW, I also have and it treats me VERY well while gaming)
Dell 4K S2817Q / Logitech Z-680 / Func MS3 Mouse / Corsair K70
NZXT Source 530 / i7-3770K / Corsair H100 / ASRock Z77 Extreme4 / SoundBlaster Recon3d
Samsung 850 PRO 512GB SSD/Seagate 1TB/2TB/WD 2TB
4x4GB Corsair Vengeance 2133 / PNY GTX 980 ti/ Seasonic M12II 850W