Any system that can handle current PC video games should be able to handle
PowerPoint well.  These tend to be upper end machines with 4 Gigahertz CPU's
(Pentium scale) and lots of RAM (2 or more Gigabytes).  As for video cards,
I kinda like ATI'ds top end ones.  The mobile Radeon x600 has done well by
me.

But there are some things you can do to help even the best system to run a
show better.

1) Reduce the resolution of the images to the appropriate size.  I know this
sounds wrong, since you want the highest quality images, but trust me on
this.  Most projectors have a 1024x768 maximum resolution.  Keeping images
that are more detailed than can be shown on the screen at this resolution
(factored by image physical size vs. size on the slide vs. screen
resolution) will slow down the handling of the ppt file without gaining you
any sharpness.  Optimize the images for the presentation.

2) Turn off the network connection(s) and disable the virus scanner.  Virus
scanners do not seem to understand PowerPoint's need to read and re-read the
ppt  file.  Therefore they scan and re-scan those files taking away precious
CPU cycles and hogging the disk access.

3) Shut down any un-required processes on the projection machine.  Again CPU
cycles are wasted on programs that are not needed for the show.

4) Always run the presentation from the hard drive, NEVER EVER from a CD,
DVD, or a USB drive.  These all have access time problems which will cause
the presentation to run slower.

5) Preload the presentation and run it thru to completion, then (with the
show still active), hit 1 {enter} to return to the first slide.  This will
cause all the images to be moved from the hard drive and placed into RAM
where access times are much faster.

There are more ideas that folks will jump in with, but these should get you
started.