It depends on what you mean by "internet access".
If you only mean "web access" (http and ftp - yeah, and gopher) then the answer is "no", as stated by stoney5050. You only have to configure the server as your proxy on the web browser.
However, if by "internet access" you mean access to other protocols (mail, news, NTP, Messenger, Kazaa, etc.) then the answer is... it depends! If you configure ISA as a gateway (as AdMcG suggests), by enabling IP Routing when enabling Packet Filters, then you don't need the firewall client... unless you need authentication. If you need to give access to some but no access to others, based on their username/password, then you need the firewall client. If you don't, then just enable IP Routing and treat your firewall as a router to the Internet - by configuring it as your default gateway.
Regards,
Paulo Meireles