Configuration for Multiple Instances of Parallels6 on IMAC

Hi, I run two or more instances of parallels on my iMac (2.8GHz Intel Core i7, mid2009 iMac). The applications I used are mainly spreadsheets and calculation stuff. No intensive graphics or games.

I recently increased the amount of physical memory to 16GB in an attempt to improve the performance and upgraded to parallels 6. The parallel instances are installed with Windows XP

Questions:
1. Does Windows 7 perform better to warrant an upgrade from WinXP Pro?
2. Any suggestions on the optimal configuration for the parallel machines? I currently set the memory on the parallels to be either 2GB or 4GB but didn't notice a great deal of difference
3. How about the number of cores?

Many thanks for your suggestions/comments.