If it's brand new, it will probably come into the second highest group. (An additional group, at the top of the scale, was introduced for vehicles registered from 23rd March onwards).
A petrol 325i Coup� has a CO2 emissions figure of 203g/km. That places it into Band F (�205 per year tax).
To find the emissions figure for the model which you're considering, go here:
http://www.vcacarfueldata.org.uk/
Once you know the figure, go here and refer to the table headed 'Petrol Car TC48' or 'Diesel Car TC49', as appropriate:
http://www.direct.gov.uk/en/Motoring/OwningAVe hicle/HowToTaxYourVehicle/DG_10012524
Chris