Get the new version 0.9.8.0827 from codeplex
The previous version of WSPBuilder 0.9.8.0822 was a memory eating monster, now the memory foot print has been greatly reduced to a more acceptable level.
The reason for this was that Mono.Cecil uses a lot of memory to load the assemblies. Therefore when loading reference assemblies and base class assemblies like System.Web the memory consumption exploded. Now every assembly gets unloaded right after it has been used.