The more Apps you have running naturally the more data you will use. Most of the Apps need an Internet connection although in their Settings you can often change the Notification rate. The less frequent these Apps update, the less data you will use. However the default Apps, even when set to frequently update every few minutes, do not use enough data to worry Orange users.
Like I said, if you download the 3G Watchdog from the Market, it will monitor your data usage and let you know when you're close to the limit.
But as I have said before Orange will not penalise you if you do go over the limit point blank. The Fair Usage policy is there for offenders of the usage limit, so if you're abusing it by using the device as a modem or downloading multiples of the limit then you will get noticed.