Welcome
If you installed apps on the SD card, you could just copy the files from one card to the other using your PC. You can use your Treo also, by just copying the files onto a temp folder on your Treo.
I can't remember, but I think WM5 (Windows Mobile 5) has trouble reading anything above 2Gb. But I believe there are a few that successfully used 4Gb. I may be wrong. I would buy a 4Gb locally and if it works well, return it and buy one at a lower cost online.