That's not a lot of devices sharing a single wireless access point. One router is fine, its your internet connection that will be the limiting factor for online performance. 802.11g will realistically provide about 30 megabits of throughput, which is probably at least 3 times your available bandwidth at your internet provider.
Even with one or more of the machines making heavy use of the internet (for example, my upstream bandwidth is swamped when running an online backup), the other machines are relatively unaffected for browsing, chatting, etc.
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-Scott
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