I actually tried this firmware upgrade yesterday on yet another G604T that I bought that day. It is straightforward but it didn`t work!
I spoke to D-Link who said that I shouldn`t need it anyway - only if I was having problems which I`m not. I did ask them if it would make any difference to wireless range etc and they said no. It would only make a serious difference if for example the adsl line was dropping.
Are you still having problems?