by default windows server can NAT/route without the need for additional ISA/proxy.
However ISA server allows you an abundance of scripting manipulation to control whom and what and to a point when. However ISA server uses an SQL database engine and it can hammer your hardware at peak times.
How to use the two nics?
Internet/public nic has your internet details, usually public nic has ADSL router address as gateway and primary dns and ISP DNS server address as secondary, with file and printer sharing disabled.
Internal/private nic has internal IP adderss with no DGW...etc.
Publish internal nic IP address as default gateway for your clients in DHCP.
Time control of access - the Draytek ADSL2 routers tend to be the best mid-priced routers for scheduling internet time I cannot remember how precise it is I.E. down to user/ip address level otherwise (and I offer this hesitantly) the BT 2 Wire which has mac address and IP address filtering for internal PCs with comprehensive time restrictions. They tend to die after couple of years though :) so keep one spare.
ISA server can kill your network completely if you do not configure it properly consider that before installing it. If you opt to use ISA server - post a time/user internet access configuration question in here so that you can get an ISA server pro to help you.
Hope this helps
