From the H60 to the H80? That's pretty tough to say for sure. From the H70 to the H80 I would expect about 4~ 5c. I have never compared the H60 to the H70, so I couldn't be too sure. If 4 or 5c is important to you, then go for it. If not I would stick with the H60. As for the H40, it looks OK but the warranty is not as long as Corsair's other coolers and since it uses rubber tubes evaporation may be an issue after a few years.
The H60 is only $10 more than the H40, and I would rather have an H60 over an H40 myself. The H40 looks like an H50 or H60 rad with an H70 pump/block. The H60 has a newer designed pump/block, so I would rather have it (plus no rubber hoses). If space is tight and you need the flexibility of rubber hoses, then you could try the H40.
If you're looking to upgrade, why not sell your H60 to your buddy at a fair price and use that to offset the cost of an H80?
Here is a good comparison of the H60/80/100:
With his setup and a i7 overclocked to 4.8GHz, he got a ~5c difference between the H60 and the H80, which is not bad.
One last thing to consider. The H60 comes with a 4-pin PWM fan (I believe), which you can plug into your CPU_FAN header and allow your mother board to regulate the fan. The H80 comes with a fan controller in the block, it will operate the two fans in any one of three ranges which is OK, but it will not give you as great a regulation as the CPU_FAN header. Of course, you could come up with your own way to run the fans if you like (such as using a aftermarket fan controller, or using your motherboard (if it will control 3-pin fans), or purchasing a pair of PWM fans and a PWM splitter to let your motherboard control the fans the same way your H60 fans are controlled). If you don't mind the way Corsair regulates the fans with the block, then you should be happy setting the fans to one of the ranges using the button on the H80 block. Most people are happy using that method.