The need for gigabit depends entirely on the content that is being pushed around the office network. Its not really necessary for anything other than moving enormous files in a shorter amount of time. Any sort of streaming content (HD video from the web, for example) should be fine with 100Mbit. After all, its not likely that your internet connection exceeds 100mbit, or ever 25mbit.

In addition to VLAN tagging you'll probably want QoS and PoE.