All, I read this thread with interest having updated my D2 to DD-WRT recently. Before finding this thread.
First I want to help clear up a few issues, so if any of the following helps take it.
I didn't flash mine 2x first with factory then with the secondary bin. Just did it with the dir615d-revb-ddwrt-webflash.bin and works fine.
Can only assume factory flash has a modified header to allow D-Link fw (not VM fw) to be uploaded to it. Guessing non factory flash is ok with VM fw or an older DD-WRT fw.
Current Firmware Version DD-WRT v24-sp2 (03/24/10) std - build 14144
When I was on orig VM fw. I had a major problem. There were up to 6 wireless devices linked to the router and it needed to be rebooted at least once per day. It would slow to a crawl and only a power down and up (not software reboot) would fix this with 6 wireless clients.
When it was used with 4 or less clients I hardly ever had to reboot it. Take this for what you will.
Other speed issues occured at the same time. When the household went up to 6 devices I thought it was a good time to update from 10Mb to 20Mb package. This resulted in the average speed being lowered from 9.8-9.9Mb to around 7Mb with occasional and I really do mean occasional bursts into the 12Mb range.
After many frustrating calls terminating in India I finally got hold of 2 Uk based engineers who actually did some tests and stopped telling me they were "refreshing the line" as was all I could get out of India.
The detail that I got regarding my slower line was that the UBR that the 20Mb package ran on was having intermittent faults and that was the cause of the problem. The solution was to drop back to 10Mb which I did and it fixed the problem immiediately. No problems since hitting 9.8 on speedtest.net.
When on orig VM fw and on 20Mb with UBR problem. I had a problem mentioned in an earlier post in this thread. Namely that clicking on a link was no guarantee that things would happen. Needed a few clicks sometimes before the next link would even start to load. Even when the router was just reset. Had a fully stable 54Mbs link. During intermittent and good periods.
I'm not saying that a dodgy UBR in your area is the cause, and if you suspect it is. Call VM in the morning to be guaranteed a connection to a UK engineer. Who ever you get be polite. I work in a call centre and it's not fun talking to a ranting punter.
About to upgrade both 615's to new firmware. After upgrade never arrived, I placed a phone call to say it didn't arrive. The agent sent another and they both arrived on the same (next) day.
With DD-WRT I'm able to use the 2nd router to make a wireless bridge (used DD-WRT Bridge repeater
, with no "virtual interfaces" as I couldn't get a Client Bridge to work)to the first on orig VM fw and have a wired network on the 2nd floor. With a very stable link after I upped the output power to 160. Able to stream HD media through 2 Victorian house interior brick walls, to an xbox 360 using it. Theres tons of guides for setting up xbox 360 to media centre on XP, Vista and Win 7 so search for it instead of asking me for help with this.
Guys I wish you all the best of luck and if my advice helps tilt a glass for me.