I must say I'm not familiar with your new router, I just trusted on Devo here, he knows a thing or two...
Install DD-WRT to your old router then, cheaper than that it won't get. ;-)
In general on Buffalo routers the WDS thing goes like this:
1. Log in to your routers GUI, by default that's on http://192.168.11.1 username "root" and no password.
2. Go to "Wireless config" then "WDS" and tick it on.
3. Select the mode from a dropdown menu (Slave EC in this case).
4. SSID is your downstairs routers wireless network name.
5. Wireless authentication is your downstairs routers wireless encryption mode.
6. Wireless encryption is your downstairs routers wireless encryption type.
7. Key is your downstairs routers wireless password.
8. MAC address you don't need in slave EC mode.