I introduced this in one of my forums and it is become one of the most popular features on the forum although the forum is vBulletin this feature if incorporated into XenForo would complement the existing notifications very nicely.
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Truly how long is a piece of string ? Unless your site is fantastically popular honestly not much one or two gigabytes of traffic is probably a good starting point. Upgrade from there, you can get bandwidth and hosting pretty cheaply anyway - so don't sweat it too much.
I have an existing forum running with the above statistics but I am thinking of starting a sister community which I know will grow very quickly and I've been tossing between using vBulletin (not happy with the path they're taking) and XenForo which is appears to have a lot of potential for the...