Upgrading to 2008 is worth it for your situation. At my last job, we had 50 projects, and VS 2005 was dog slow. Plus it was a pain to debug (w/ multithreading). The IDE kept blowing up on us. As soon as we upgraded to VS 2008, everything went faster, the compiles went faster (partially because MS implemented a smart parallelization algorithm by the looks of it), it uses less RAM, and is far more stable.
The only regret we had with 2008 is in the C++ end of things (most of our app was VB.Net, some C#, and some C++). The new version of C++ doesn't support Windows NT which we needed to do. The workaround is a postevent that uses an old Edit Bin and sets the version for 4.0.
