i had vonage for a while before comcast switched over to their own digital phone service.. ours came as with big modem+phone.. but there is also the router version.. i dont care much for the router version, pita to deal with.. but i never had a dropped call.. you should still be able to use your dd-wrt depending on how you setup the vonage router.. if you are able to set it up as an access point then you should have to get rid of the dd-wrt but thats a total if since ive never done it..
the sound quality is so much better with broadband phone.. after having it for 3 years now i dont think i could ever go back to using a regular lan line phone..
The new Vonage router (VPortal) is much smaller than their older boxes. I used Vonage for about 2 years then switched over to another provider when it looked like Verizon might sue them out of existence. Since that cleared I switched back to Vonage 4 months ago or so... no real problems, nor did I have any the first time around with them. Different broadband providers have different policies in place regarding whether they honor packet prioritization (Comcast being one of the worst, supposedly) although I use TWC and have had no issues.
As for gaming I've seen no difference in throughput or lag when using the VPortal vs. using just my cable modem.
Also I have my Linksys WRT54GS wireless hanging off the back of the VPortal with no issues, even when using VPN access across the wireless access.