i think capita are saying that as of 1 August you will HAVE to be on sql2005. this update, and all future updates, will only support sql2005. so any practice that you can get on sql2005 will be invaluable come the summer. however, if your requirement is urgent then the current release, of sims, will work on sql2005 but it is NOT fully compatible and capita will not support this environment.
my understanding of licensing, which i admit is sadly lacking, is that if you've paid your money then you can install any version that you want. so you may have to pay again for the sql2000 media but i would definitely keep hold of the sql2005 media as you will need it very shortly. obviously, you will have to purchase additional sql licenses depending on what you have already purchased.