I don't believe there are any 8GB SD cards yet. According to this site:
http://www.sdcard.com/usa/TextPage.asp?Page=28GB SD cards are not due until 2007.
Now the reason why some cameras and PDAs can't read greater than 2GB SD cards is because they can't read FAT32. Since the GP2X can read FAT32, it should ideally be able to read a 8GB SD card when it comes out. Indeed, I bought a 4GB SD card yesterday and it works perfectly fine with my GP2X but can't be read by my Casio Z750 (even though it reads 2GB SD cards fine but it can't go beyond it since anything greater than 2GB will automatically be in a FAT32 format).
In either case, when the 4GB SD cards first came out during last summer, they were hundreds of dollars. Now you can find them as low as $149 at eBay or a few dollars more at newegg.
By the way, my 4GB SD card is an A-Data MyFlash 150X and, again, it works perfectly fine with my GP2X.