If you are willing to use the Python programming language, I wrote a libary called DecInt specifically designed to calculate very large numbers in exact decimal format. It will use GMP if GMPY (GMP for Python) is available.
For Python, go to
www.python.org
For DecInt and GMPY, go to home.comcast.net/~casevh
For psyco (a Just In Time compiler), go to
http://sourceforge.net/project/showf...group_id=41036
Using an AMD Athlon MP2800+, I'm able to calculate M43 in under 7 seconds.