(I now its way past but It should be noted)
If you have a subscription to XBOX live (Silver or Gold) you can still play "compatible" Original Xbox Titles that work for the 360 on Live, only on Pro and Elite Xbox 360 Consoles. Xbox 360 Arcade console dose not have backwards compatibility to play original Xbox titles.
that and EA games from original Xbox do not work for some reason
you can get the subs by these means:
1.)Transfer your original Xbox Subscription over to your 360.
If you have a trial card (they still work for some reason , sometimes) , or a bought in store Original (Orange black and green colored card) Xbox live card.
You use your cards on the OrigXbox, get it all set up. make your gamertag. then save it to the OrigXbox (OrigX for short). Take your 360 and go To your live accounts, there should be something about transferring your origx live account over to your 360 live account.
then after that you'll need to delete your live data from OrigX's HD
way #2 is: Xbox Live Silver Or Gold Cards ! can be bought at most stores silver has less features than gold, thus it can be found cheaper in some spots.
basically the same as the OrigX, Cept with the 360 instead.
3. Trials! (Like free samples at Sam's club) 360 comes with Silver in the box , you can get on live right out of the box. plus some games have trial cards in them for live.
4. credit card, no explanation needed here.
5. Bill by mail , with checks and stuff. This option gets set up by phone, really lame.
that's all that I know, by surfing, reading and my own experience. all in all its like a really good magazine subscription.