My 6 years old PC with Intel Pentium D is due for an upgrade.

Would Win8 a much improve version of Win7?

Would current Intel i5 etc. have enough power to run Win8 efficiently?

I played briefly with Windows 8 beta and it did not excite me much at all as a new operating system. The new interface that you have seen pictures or videos of has obviously been tweaked for tablet usage. However, there was not much else that might entice me to upgrade for a PC. Unless there is more features waiting in the wings, I wouldn't be too concerned. I likely will be sticking with Win 7 for a while until something else pushes me to change.

It's pretty light on required specs so it should run fine on an i5. I'd base your processor choice more on what other software and tasks you plan to run on it.

Edit:
If you think your current PC will run it, you can try Windows 8 for yourself without upgrading by using the trial version and Virtualbox that allows you to run another other OS virtually.

Windows 7 seems to be a winner. It's similar enough to XP that there's not a huge learning curve. In my experience, it has been very stable as well. On the other hand, I haven't heard much high praise yet for Win 8, and no compelling reason to consider it. Reviewers have been panning the changes to the Start menu.
(Disclaimer: I have no personal experience with Win 8).

Windows 8 seems like they just took their O/S for cell phones, bumped it up a bit in features for tablets, and then bumped it up a bit more for PCs. I am not a huge fan of the interface on a phone, and not on a PC either. Nothing like big square blocks on the screen when there has been such a push for roundness and semitransparent overlay of widgets, etc...

Go with Windows 7 and be happy about it.

In answer to your question, yes, a modern PC will run Win8 just fine, as long as you get enough RAM and a fast enough processor, but the same applies to Win7.

I'm an early adopter

I'm a "windows" guy

I love the Metro UI concept in general (Media Center, Xbox, Zune, WP7...)

I think Win8+Metro will be great on tablets.

So far running Win8 on a desktop I think it's lame with a mouse and M$ has completely crippled the legacy desktop so I see no reason to wait for it.

As i read recently, "windows 8 is like adding a make-up kit to windows 7".

I wouldn't bother to wait.
Win7 has been fantastic so far.