The MAC address thing is a huge myth/lie, I don't know who ever came up with this. But it is not possible for Jagex to track a MAC address, even if they wanted to. The MAC address is only used for internal traffic - to/from the router/modem.
Let's take the following:
When you send data (a packet) to Jagex (a login request for example):
The first destination is your router. The packet you send has the MAC of your router as a destination and your computers MAC as "source" (where it came from). When it reaches your router, your router sends it to your ISP exactly the same way - your router uses HIS MAC as "source" and the ISP and destination. So and and so on, your ISP will route the packet a few times the same way before it reaches Jagex.
Meaning your computers MAC will NEVER leave your internal network, and the MAC of your router will only GO AS FAR as your ISP and never beyond the first piece of network equipment of your ISP.