Well i haven't tried this with mandriva 64 yet, but the main problem i run into is that on 64 bit systems you dont have the w32codecs package. You can use w64codecs, but that one misses a lot of codecs compared to the other one. Either you have to crosscompile (which i tried and failed at a couple of times) or just give up on 64 bit systems.
Since my system does not have over 4 GB of memory there's little point to a 64 bit system anyway. It would be nice to make full use of the 64 bit processor, but it seems microsoft's proprietary codecs are not coming to 64 bit systems anytime soon, maybe we'll finally get a spinoff now since 64 bit processors are pretty much everywhere now and with w7 other companies finally decided to support their hardware on 64bit. It'll happen, but it just takes a while.
