If your computer has a DVD player I would go for the DVD version. I do not see a CD version for TOPO U.S. 2008. In the long run I believe it gives you more bang for the buck.
http://www.gpsnow.com/gmmstp.htmSystem Requirements: PC with Windows 2000 or newer operating system; 32MB RAM minimum; 2 GB free hard disk space; DVD drive for DVD versions; available serial and/or USB port, 256 color display adapter and monitor (16- bit color required); and mouse or other pointing device; PC Interface cable, Internet access is recommended
I intentionally deleted mention of a CD player in the system requirements because I do not see a CD version offered. While DVD players have become common, many older computers do not have a DVD player.
Micro SD cards in the 2 GB version are becoming cheaper. On sale they can be purchased for $25 plus tax for the State.
Should you have questions on loading MapSource Data from your pc to your gps I have a very long how to do it which can be posted. In fact I will post it as a new topic.