Setting up a PC to a WAN port for Router bandwidth testing

I need to test the bandwidth capability of a Router for troubleshooting reasons. It appears the Router is chocking on higher bandwidth downloads as this was suggested for me to try this. I never had need to do this before and have little idea other than the basic re-cabling connections.

Situation;
Two Towers, both running Win 7 Pro; I'll call the WAN side PC A, the LAN side PC B
MB's have the same AMD 970 chipset, one tower has a AMD FX8350, the other has a AMD FX6300
Router is a TP-Link TL-WDM3600 (Gigabyte) running openwrt firmware
Both are in the same room (for convenience) with 6' Cat5 (or 5e) cables.

Other than setting 'Network Properties' of the PC B to Auto, what addresses do I set PC A to?
What and where do I change in the Router as to addresses and DHCP settings?
Should this be a DHCP Client or a Static address?
What else do I have to address oin the Router?

Lastly; am I accessing each PC via a web browser or just by 'Explorer' like I would normally?

For a start what is your connection is ADSL , cable ? It's not clear what you hope to do. In most cases you only get one internet IP so you can't run a PC on wan and another on LAN as that would need two IPS. You could connect a modem to test the speed on its own.