My 2 (unsolicited) cents
I tend to agree that most newer routers (Asus in particular) can do QoS pretty well, but for the peace of mind, get a router (and a few of the suggestions above N12, N16 fit the bill) that supports Tomato and you should be good.
DD-WRT seems to be behind Tomato in QoS performance - but it's also a choice, as it will work on more cheap routers (I just got a dual-band Belkin for 25$ and it runs DD-WRT).
My main router is a Linksys E4200v1 with Tomato Shibby and never had any issues with simultaneous voice calls and P2P traffic.
Having a good broadband connection also helps.."throwing bandwidth at the problem"
PS Maybe the topic should be moved to another subforum as it does not deal with Obi firmware.