Never heard of "Standard" or "Dual" speed switches, perhaps you are looking at hardware that is more sophisticated than you need. A wired network running at 100Mbps is the norm.
<<<How will port forwarding be affected by having a switch connected to the router?>>>
Port forwarding is set up in the router to forward the internet traffic to the relevant IP address in the LAN. The switch you install will behave in exactly the same way as the router's built-in network switch (its four LAN ports) and won't have any effect on port forwarding.