thank Yii. Appreciate the quick response.... however living in Poland, Europe we dont come across names like Optima, Douglas etc...

What about Bosch or Varta ?

Here is what i have learnt so far from the forum:

1) From what i gather the specs need to match exactly ?? So for the x5 3.0d 2005 the battery i need the equivalent of the below:

a) Original BMW battery, filled 90ah part number 61218381768 or
b) Original BMW AGM-battery 90ah part number 61216924023

can anyone confirm this ? Whats the difference between a&b in real pratical terms?

3) Then if i was to buy an alternative it would need to be the same specs:
12 V / 90AH / 720A

can someone confirm this ?

4) In that case would the following Bosch or Varta with same specs be ok...?

BOSCH BATTERY 90Ah 720A S3
S3013 / 017 BOSCH Car Battery 12V 90Ah Bosch Car Batteries Free 24hr UK Delivery

or

VARTA BATTERY 90Ah 720A
Type 017 Varta Black Dynamic Car Battery 12V 90Ah (Short Code: F6) (Varta DIN: 590 122 072)

5) I also read something about an air vent...? does anyone know if the above have an air vent ? If not then should i not use them?

6) Any other recommendations would be much appreciated...