Your plan will work without any problems or issues if you use your existing SIM when you complete the activation of your new iPhone 3G. Just tell the sales rep what you want to do.
If they insist that you get a new SIM then you will have a problem when you sync with iTunes. The original iPhone has to be paired with the SIM and this is done using iTunes, so you can just reactivate the original iPhone using the new iPhone 3G SIM. Then put the same SIM back in your iPhone 3G and call Customer Care, they may ask for the IMEI of your new iPhone 3G, just give it to the rep and ask them to switch you back to the iPhone 3G $30 data plan. Once this is done you can freely swap the SIM between your, iPhone 3G, original iPhone and even your Razr.
Others will tell you to just stop once you reactivate your original iPhone and stay with the $20 original iPhone plan that includes 200 Txt SMS text messages. This is a violation of the AT&T Terms and Conditions but you get to decide how you want to proceed.
If you have more questions, just provide us more details and we will give you additional suggestions.
Dave