Hi,
Easiest way to check is: Lift boot liner, remove spare tyre tooltray. Look at sticker in boot. This will tell you what engine it is (or supposed to be. AGU = 150+ko3 turbo, AUM= 150+ ko3s turbo, AUQ=180 + ko3s turbo.
Ko3s will allow you to gain more power through tuning. AGU's will be earlier 98-00 IIRC, AUQ will be 2002 onward i think and in between is usually AUM.
Anniversary golfs will always be AUQ but some of the later standard gti's will have the AUQ 180 BHP too. Only the anni's differ in appearance, apart from some factory options (spoiler, xenons etc) but have held there value better so expect to pay 5.5k upwards.
You can also verify the engine code as it is stamped on the block and easily accessible.
HTH Sam 
thanks Sam, fully armed with all the information I need now thanks, now off to find one advertised at a realistic price on AutoTrader is the next hard part...
I have a 180bhp AUQ non Anniversary model. As said above, the quick way is the 6 speed box but the other way to tell a 180bhp over a 150bhp is the rear brake discs are vented

Other than that, they are the same. The sticker in the boot says 132kW (180bhp) rather than 110kW (150bhp).