So are you wanting a new computer or are you wanting to upgrade gpu/psu? If you want a new prebuilt computer and your budget is 1550 +350amazon then we can work with that. note On cyberpowerpc.com I can build a better computer with a different case but better for 1660ish after shipping, rush on it and promo code "instant". and it has shogun 2 total war and dungeon siege 3. 100 over that budget. offers expire tonight. and realistically you don't need an i7 you could get the i5 for about $100 less which would put this in that 1550 range.
Edit You could upgrade the psu and the gpu and it will be able to run sc2 on medium settings/high and civ 5. I had a core 2 quad q6600 with an 8800gts and I was able to play sc2 (meduim/low settings) and civ 5(only tried single player).