Ask a more relevant forum. State your uses, and just say Dota 2 as a game.
What this means is that basically you just want the fastest possible solution for abode CS. Dota 2 will run fluently from anything that is midrange onwards.
i7 is preferred afaik, over i5. For gaming and most use, there is no difference. RAM is better if plenty, for editing stuff.
nVidia has some exclusive computing features. However, it seems that the computing performance on the GTX 680 is pretty crippled. Otherwise it is an extremely fine card, fastest single-gpu, pretty silent + cool and supports more than 2 screens.
GTX 580, I have one. Right now, if I bought one, I'd buy it used... rather many people who sell these cards, tend to sell the one and then upgrade for the next high-end for around 200$/€.
They are a bit noisier and provide more heat, but it's manageable with custom cooling solutions. The real annoyance might be that it only supports up to 2 screens.
Then there are AMD solutions, HD7970. Doesn't have the nVidia advantages, but they aren't going to restrict features from their cards, at least as much.
It seems that nVidia has made GTX 680 purely a gaming card, and wants non-gamers to get a quadro instead. Which costs a ton.
Take it to appropriate forum, the baseline computer would be something along the emperorguy's suggestions.
Main issue is to get the best performance for your computations, and knowing exactly how much of it you need. Maybe you only need CPU performance and a poor GPU would be enough; in that case a current midrange GPU would be great since it is enough for Dota 2.
Also, take into consideration whether if you use more than two screens or not.