You will get 512MB more memory as the graphic card 512MB is included in the 4GB pool, might get more useable memory to as I the 4GB limit restrict you from using swap file.

The real benefit is that it gives you the option to upgrade to more than 4GB,  

I also found 32bit poser pro 2010 pretty unstable on my old XP system but it had issues.  

 

Main downside is that you have to reinstall windows, my advise is to go 64 bit then you have to reinstall anyway or wait until you buy more memory.