I want to upgrade my internal memory card from 2GB to 8GB. I was planning to put both cards into my computer and use Windows Filemanager to copy the folders and files from the 2GB to the 8GB. Then I would install the 8GB and increase my virtual memory to the max.

I have some maemo mapper files on the 2GB card, backups, etc. Is this a good approach??

I have no idea, never tried to do this. But if you copy the files and try it, you should be on the safe side, as you aren't doing anything to the original 2GB card.
Try, run the apps that use the files, and let us know how it went.

If you have a N800, and you're not booting from a SD card, then the process is as simple as copying all the files.