I picked up this piece of advice from another forum...the advice was to keep hold of the original 512MB of RAM rather than sell it, as if you need to return the mini to Apple, they will want to receive the computer as shipped (in terms of hardware) otherwise you may find that your warrantee is void (or is it voided?). I guess that once the warrantee expires (1 year?) you will be free to sell the unwanted memory.
I'm waiting until late August to buy my mini - the Apple World Wide Developers' Conference is taking place at the start of the month, and I'm hoping for some upgrade announcements (chip speed etc). It may not happen, but I can wait.