SD cards come in classes. The higher the class the faster the card can read/write. So if your SD card from your trusty G1 is fast, expect to the same performance on the Vibrant.
I do not know the R/W specs for the Vibrants internal storage off the top of my head; however the internal memory on the Vibrant could be faster than your memory card so you could end up having lower performance if you move your apps to external storage.
I hope that helps