Intel Core i5

All of the i5s which support Hyper Threading are the i5 6xx and are dual core versions, meaning that there are 2 physical cores, which act as 4.

The Quad core i5s, ie. the 7xx series, are quad core versions, but do no support hyper threading, meaning that they have 4 physical cores, which act as 4.

If you are looking for a quad core processor which has hyper threading, you have a few options.

Firstly, if you already have a board which supports the first generation i3/5/7 series, with an LGA 1156 socket, you could look into the i7 8xx series. All of these are quad cores with hyper threading. This was the first line of i7s to be released.

Secondly, you have the X58 i7s. These are lablelled the i7 9xx. The first of these is the i7 920, which runs at 2.66Ghz on 4 cores with hyper threading, but if your budget allows, you could stretch to the i7 990x/995x which run over 6 cores at 3.46Ghz and 3.60Ghz respectivley, but with hyper threading, act as 12.

All these chips require a socket 1366 motherboard with an Intel X58 Chipset.

The final option is the new Sandy Bridge approach. In this series, you will need a Socket LGA1155 motherboard, with either an Intel H61, H67, P67 or Z68 Chipset. You can then combine this with either an Intel i5 2500/2500k, or and Intel i7 2600/2600k. Both of these are Hyper threaded quad cores, and run at 3.3Ghz and 3.4Ghz respectivley.

If you are an overclocker, you will want to look at one of the two latter chipsets, and the 'k' version of the processors.

I hope this has cleared things up for you.

It depends on what type of i5 you wan't. I've currently got the i5 450M. There are faster and slower versions of the i5. The reason why there are i3, i5 and i7 is due to the used transistor and chips. The i3 is a powerful CPU but it has older and cheaper transistor and chips. The i7 has faster and more expensive transistor and chips to make it (but it's defiantly worth it!)

the i7 does not run on 8 physical cores, it runs between 4 and 6 cores depending on the model. and yes it has hyper threading

Currently I believe there aren't any Core i5 processors on the market which support Hyperthreading. You can either pick up a dual or quad core Intel Core i5 and that's about it. If you want Hyperthreading you can pick up a Core i7 that supports it although Hyperthreading isn't actually having 8 physical cores but instead 4 physical cores and 4 virtual cores.