Yeah, 64bits rock!
When I changed my OS from WinXP to XP64 I had three times the speed with the same hardware.
You should really use a 64bit machine when you simulate structures larger 3GB. I am not sure about HFSS9, but HFSS10 has the 64bit solver included. The usage is the same, simply install xp64 and HFSS. It will automatically use the full power.
HFSS11 has a new solver that seems to be much faster than the MCA from v10. If you are making Metamaterials or related things the version update could help you.
Alternatively, you have the option to reduce your model. Simplify the structures and use symmetry-planes if possible.
I just set up a simulation server with two 2.66GHz Xeon double core processors and 16GB RAM with Win2003Server64 and HFSS11. Hell, thats fast!