In VS2008, you can hit CTRL to make an intellisense popup partially transparent so you can read the code under it.
Is it possible to adjust the degree of transparency somewhere? Like a registry key, for instance? I find the default translucency a bit too opaque and still struggle to read the code underneath.
If all else fails, take the source code for my GPL VS add-in and modify it to your needs. One of its features adds this ability to VS 2005 using the same opacity as VS2008 uses. By default it's conditionally compiled in the VS2005 dll (SamTools2005.dll), so you'll need to compile it back in to the 2008 one using a modified opacity.
The source installs to the program files directory, or you can use a program like 7-zip to extract the .7z file from the msi installer itself.
SamTools features (which for some reason doesn't list the opacity modification)
SamTools releases