Even still, it isn't worth downloading a huge framework (upgrading from 2.0 means about 120 mb, upgrading from 3.0 means 50 mb, and starting from scratch is about 200mb) for one or two plug-in's, besides, the new VS allows you to compile class libaries for .NET Frameowkr 2.0, 3.0 and 3.5, the only new features you'd need are in PDN itself, you wouldn't need any of the .NET Framework libaries, apart from the drawing tools which aren't needed for PDN 3.20 because of the IndirectUI system.
at my job I did a test where I used VS2008 and compiled my solution targeting .net 2.0, the assemblies it generated were in total about 15% smaller than the ones that were generated from vs2005
I read in a programming blog that even if you target .net 2.0 in vs2008, it still uses the newer compiler, so you still get a benefit from all the lessons they learned and compiler optimizations from vs2005 to vs2008 while maintaining compatibilty.
just food for thought