Hey Geoff,
As a short overview you can get the:
03-06 models which are 276BHP
06-07 models which are generally the revup facelift models at 296BHP
07 onwards which have the 313BHP new duel air intake fancypant engines
All of these models you can buy with or without the GT pack. The facelift model also brings with it a few changes like rear LED lights amongst a few other subtle changes. There are also JDM cars which can be found at very reasonable prices just now. Powerwise there is very little difference between the cars so its really down to the spec, age, condition and milage you feel comfortable with.
Read the buying guide on here - it will tell you all you need to know!