hello we went with online mode for a couple reason:
1) it is hard to predict the space required b/c we dont govern mailbox size and every user is different
2) cache mode relies on a local copy of the mailbox which will increase i/o
we implement online mode using GPO. it's relatively easy, if you can't get it going reply back and i will point you to the required GPO setting.
what version of outlook are you rolling out on your golden image? we went with outlook 2007 and there are some special considerations you must take into account if using outlook 2007 + online mode in regards to advanced searching.