If you run the DownloaderDiv_EN.exe program again you can check the download location without needing to start the download again (it has been a while, but I'm pretty sure it remembers the previous location if you change it from the default, though if you did change it you would probably remember where).
In XP the default download location is
C:\Documents and Settings\ %username% \My Documents\LarianVaultDownloads\Div_EN
or in Win 7
C:\Users\ %username% \Documents\LarianVaultDownloads\Div_EN
There are some compatibility issues with the disk version saves in the download version of DD (some people have reported that the game crashes trying to load the disk saves, or it will load them, but then saving the game afterwards will be disabled). If the disk saves load and you can still save, then you should be fine continuing with the same game.
The Larian Vault versions of DD and BD are the same as the current GOG versions (don't think GOG has released a recent patch for BD, but there is an alternate download for it in one of the GOG forum topics). The 'HD' just means support for widescreen resolutions and higher standard resolutions.
There is no official patch for the original disk versions of DD or BD to add higher resolution support.
BTW, as I mentioned in a reply to your GOG forum post, there is an update for the GOG and Larian Vault version of DD (and the anthology DVD version) that hasn't gotten an official patch yet:
music fix. I don't think the main Larian Vault install program has been updated to include this yet, either. Check the version number in the bottom right corner of the main menu, and get the update if it is less than 1.0062 (if it hasn't been updated yet it should be 1.0061).
