You're not telling which platforms you are familiar with. That partly influences the choice. My basic range would be
1. Linux-based firewall (you can reuse old 486 PC!), _if_ you're familiar with Linux.
2) Appliance (black box), e.g. Nokia or Cisco Pix. Managed through a web browser usually.
3) A Windows-based fwall, if you really want Windows. Most products would do, e.g. Watchguard and CA e_Trust; but usually _not_ Microsoft ISA which is really the worst of the bunch, especially in growth scenarions. MS itself (SQL development unit) decided not to use it for its internal LAN but go for an appliance...
HTH,
- The Netherlands