If you already have Skyrim on Xbox, my recommendation is to save up your money unless you are really into using mods.
And believe me, searching and playing with mods for an Elder Scrolls game in itself is a very enjoyable activity. I call it "Excessive Compulsive Moderitis"

My recommendation is to buy Diablo 3 first and enjoy it, and once you're done with it (provided you're not the hardcoe type who wanna keep grinding for gear)
Dawnguard should be out for the PC and i think there will probably be another Skyrim sale too.
Just my 2 cents.
edit: WIth regards to your PC specs, you should have little problem in running vanilla skyrim or even playing with mods.
You might wanna get a beefier gfx card if you wanna extreme mod skyrim using ENBs to almost photographic epic movie like quality.