What hardware improvements should we make to speed up our build machine?

We have a build machine running in our development department, which we've set up to build continuously throughout the working day. What this does is:

• Deletes the source code previously checked out (5 minutes)
• Does a clean checkout from subversion (15 minutes)
• Builds a whole bunch of C++ and .NET code (35 minutes)
• Builds installers and run unit tests (5 minutes)

Given the above, what sort of impact would adding different hardware have on improving the time it takes to do the above?

For example - I was thinking about using an SSD for the harddisk as compiling involves a lot of random disk access.

The subversion server is currently a virtual machine - would switching it to be a physical machine help the slow checkout?

What impact would upgrading from a Core 2 Duo processor to an i7 make?

Any other suggestions on speeding up the above?