Linux Mint 15: Share internet connection with subnet

Linux Mint 15: Share internet connection with subnet

I want the following setup:
Internet ---- Cable modem
|
|
+--- Router A ---- PC
.
.
(wifi) . . . . [PC, tablet, etc.]
.
.
HTPC Linux Mint 15
| +---- TV
| |
+--- Router B ----+---- PS3
|
+---- Receiver
I currently have internet access at HTPC through wifi from Router A.
I want to have internet access for HTPC and devices after Router B.
As of now I don't care that devices after Router B see devices before
Router B.
I would like to know how to get internet access for devices after Router
B, while keeping internet access of HTPC. HTPC might occasionally pull
internet content through OpenVPN.
I tried [Networks Settings] => [Wired] => [Options] => [IPv4 Settings] =>
[Method:] = "Shared to other computers" on HTPC Linux Mint 15, but it
didn't work. I don't know anymore where to look to find a solution, if
any.