Going to answer my own question here.
I updated to Windows 7(Ultimate) x64(for testing purposes only

)
The DSM connects and authenticates fine with the win7 pc (using Windows media player as media server) but I noticed a few problems.
The sound and picture quality was terrible compared to sharing with vista and after flicking through playing a few video files the player would give up and start saying the "file format is not supported etc etc". even though i had played the files a few min ago! had to reboot 520 to get working again which was quite strange.
So I installed the D-Link media server software, connected fine and played files perfectly.
one problem though when you add a folder to share it out all the files show up. but it will not add new files to the share list when i pressed the button to update list. I had to remove the folder from sharing and add it again to get it to recognize new files in the folder.
So there you go. thats my findings with win7 and the DSM520.
if anyone else has tried using win7 and the 520 let me know how you got on.