Which is better at multi-tasking? i3 or phenom 4 core?

Four REAL cores are always better than two and two hyper-threaded in terms of multi-tasking. ALWAYS. Not necessarily in games or certain applications, but if you're going to be running several things at once, go with the Phenom II/Athlon II X4s.

And, FYI - Hyper-Threading and Hyper-Transfer aren't the same thing. In fact, I've never even heard Hyper-Transfer used except when referring to HTTP.

+1 for the quad-core. Not all applications will be able to make effective use of Hyper Threading.

Hyperthreading should be transparent to the OS and programs - anything that can take advantage of a quad can also take advantage of a hyperthreaded dual. That having been said, the quad is a better choice unless the dual has a significant clockspeed advantage. In other words, I'd take a 3.2 GHz core i3 over a 1.8 GHz Phenom, but if the clock speeds were even remotely close, I'd get the Phenom every time.

Oh, and hyper transfer is not hyper threading. AMD has hyper transport, but that's different too.

Thank you all.  

Yep as said, a true quad is better.

I don't fully agree, and benchmarks will demonstrate this. Hyperthreading uses a specific set of CPU instructions. If the application in question is not able to make use of them, it will not benefit from HT.

or you can look at an i5??

Quads are better for multi-tasking. Brute force power of a quad vs enhanced power of a dual. The Quad wins.

Thank you for replying  

I meant to say, Thank you all for replying.  

No, it doesn't. Hyperthreading runs the same instructions on a virtual CPU core rather than a real one; only the OS needs to worry about where it assigns threads, the programs don't care.

Theoretically a poorly optimized multi-threaded application, which takes less time and effort to write, will benefit the most with hyperthreading.

Other than the work needed to add the multi-threading nothing else is needed to support hyperthreading. (No specific CPU instructions.)

OTOH: well optimized multi-threaded applications will not benefit very much from hyperthreading but would definitely benefit from having more real cores; in fact having virtual cores can be detrimental to well optimized applications.

Best answer selected by sidhardtha.