so here is the way i figure it. Since you were already planning to get build a new computer in the first place adding a couple hundred dollars worth of graphics cards is not going to add the price of the system too much. Also if you were planning on playing games with this computer because the more gpu power you put into this computer the better the games with play, and the more btc you will mine. Also from personal experience i bought a second 5870 for my pc to play games, and mine, and it has paid its self off already. So i would say build you computer and throw a couple of high end gpus into.
That was my thought as well...
The biggest problem here would be that I could afford to get a good machine to work on and pay in cash since I will be coming into some money this week,but adding the extra cost of say 3 more GPU's,would make it impossible to buy in cash,and I would have to resort to installments.
That was the reason I wanted to see if getting into bitcoin was worth the extra purchases now.Otherwise I could hold off on the 2nd GPU for a little while till I have the extra cash.
Also,I don't really game that much anyway,and when I do,I tend to play older games that I might have missed when they first came out.