Review the Gaming and Gamefuel sticky for one or multiple xboxes. Yes, it wont help speed, however it keeps everything prioritized and working well with more that one game console. It's been proven to work for me and others I've helped set set up.
If your using DSL then have the ISP bridge the DSL modem to the router as most DSL modems have a built in router and effects how the 4500 and game consoles NAT to the game servers. Once this is done, you'll be fine. Cable modems don't have this issue.
If your 4500 is working well on v1.21 then I would keep it going until you see problems. Some people didn't have any issues with that version, others did. Let us know how it goes.