Microsoft Excel 2003 and below is not designed to be a multiprocessor-aware program -
http://support.microsoft.com/kb/265361
When you say Excel 2007 runs slower; I think there have been problems with this (think Vista vs W7). Some people have reported improvements re-writing all code and links in 2007 instead of just opening a workbook created in an earlier Excel version. The next version of Excel is supposed to fix this
http://blogs.msdn.com/excel/archive/200 ... -2010.aspx
Just a thought...Have you tried running mutliple workbooks instead of mutliple sheets and let your OS run these on seperate threads?
My personal opinion is that it's not worth paying the extra money to speedup your setup which could all be undone with a slow internet connection, slowdown of the site you're accessing and even the software itself.
I use the entry level Pentium I7 at home and this is more than enough for anything I have ever needed.