If you have the budget for an i7, defiantly go with it...But if you want to keep the budget low and have high performance, go with the 955, it has high overclocking potential...The 940 would be also very good if you are looking for an even lower budget build, it's just that you will have to overclock it more to match the performance of the 955...
I say, chose what you can afford so you won't be sorry in the end...The 940 and 955 will come very close in performance and should fit your needs...but if you want extreme horsepower, take the i7 and overclock it...
Good luck choosing