You will be stunned by how much graphical difference enb modding can make to skyrim on PC, not to mention a whole load of new content added by other mods. I have over 600+ new spells that scales with perks and spell power armors etc. I cringe whenever I see a buddy of mine playing vanilla skyrim on his xbox.
http://imageshack.us/f/846/screenshot157n.png/
http://imageshack.us/f/7/screenshot174t.png/
http://imageshack.us/f/52/screenshot198a.png/
http://imageshack.us/f/641/screenshot224g.png/
http://imageshack.us/f/651/screenshot207pt.png/
Those are in-game graphics though some quality is lost in transition to screenshots.
You already have a decent GPU so grab a good processor, (look into intel i7 series) and overclock it. I have an intel core i7 930 2.8ghz overclocked to 4ghz with ATI radeon HD 5850 backed by 6gb of ram. Bought it over two years ago, can still handle anything I throw at it.
Whatever you do, seriously consider overclocking your CPU, that's the only way you get the most out of your bucks.
Intel core i7 930 OC @ 4.0 ghz
XFX Double dissipation Radeon HD 7950 356 bit 3gb GDDR5 OC @ 1150 MHz core + 1575 x 4 memory
Triple channel DDR 3 12gb RAM 1600 MHz
