Depends on the firewall, of course.
This computer is my old one- 256MB and 1.8GH. Ran it with Windows and ZA or Kerio (now Sunbelt) for years. Now it runs Ubuntu with no firewall as such but sits behind a router. Doesn't seem to make a lot of difference- still slow if too many apps running.
My main computer has 1GB and 1.8GH dual core. The Windows partition has Windows firewall but it sits behind the same router. Before I had the router, I ran ZA, Kerio or Comodo 2.
I don't use any other firewall because a) it's behind a router and b) I only use it for games, where a third party firewall does slow it down.
In short, I'd recommend a router with a firewall- let the router do the work. If you're not running games or high-demand applications, install a firewall if you wish- shouldn't have too much of an impact.
