Mudbox (32 bit) can take advantage of additional RAM when installed on 32 bit Windows XP.
If you are running Mudbox (32 bit) with 4 GB of installed RAM on a Windows XP (32 bit) system and you are planning to work with models that are subdivided to more than 6 million polygons, set the following in your boot.ini file to enable Mudbox to take advantage of an additional 1 GB of virtual memory beyond the 2 GB system default:
Note however that if you are working with a model subdivided to this level, it is preferable and recommended that you run the 64 bit version of Mudbox.