Videoediting accelerator cards

I need to see about getting some reasonable cost hardware cards to accelerate encoding/compression in video editing on a PC, but I have a new Mac as well.

I am shooting behind the scenes at my shoots for clients and later to be edited to the Web, and I have been known to export up to 30 minute long clips. If I am doing some good rez, I know this can take overnight based purely on my current hardware, so I am looking for suggestions. I may export 480p or better to use for www.vimeo.com

One box is a 2.2ghz 2gb ram XP machine, another is a 2gb ram 2.0ghx AMD x2 64bit Vista/XP machine, I may build an 8gb xeon based Vista/XP machine as well.