i90c Roxy

I have an old, dinosaur i205 Nextel phone that I currently use with Boost pre-paid service. I recently bought a new i90c Roxy off Ebay. It says the phone is unlocked but doesn't have a sim card. Can I just use the sim card out of my old i205 in my new Roxy phone and will all the features work etc.? I think the sim card I am using didn't originally come in the i205...I think it came out of another phone. It works with the Boost service I currently use. My apologies for the silly question, as I am definitely not a cell phone connoisseur.

Also, this is not phone related, but what do I need to do to be able to upload a custom avatar?

Thanks for any help or advice.