You'll certainly be able to play Skyrim with a 4870 on medium settings. The latest mid range cards (7850 or 7870/GTX660 or Ti) will let you play with nearly everything maxed out and a 2GB card will let you use the high resolution texture pack. Not necessary but definitely enjoyable.
The high-res pack works fine on a 1GB card. (｀・ω・´)
i7-4790K (NH-C14), 4x8GB DDR3-1866 CL9, GA-Z97MX-GAMING 5, SAPPHIRE TRI-X OC 290X, 2x 256GB Q Series / 128GB Vertex 4 / 3x3TB DT01ACA, CX750M (PSU), VG248QE, 3x M2060SWD2, MIMO 720F, Majestouch-2 (red) / Vengeance M95