My framerate depends on the pack, but I get 35-70 fps at the highest settings generally except for occasional terrain generation lag in Infinity. I've avoided optifine since I've read of the mod generating issues when used with some other mods... I have heard that it uses a second core but have no firsthand knowledge.
You won't see 4690k performance, but then you'll have saved enough money to beef up other areas that improve MC performance more, such as adding a SSD.