Here's an awesome tip for single-players who are interested in cashing in and leveling quickly.
* Diablo 3 is a one-time cost and there are no monthly fees.
* There is a party limit of 4, and the game is designed for the more difficult encounters to be handled by 4 people.
* Each player has their own drop modifiers for personal drops and magic finds. There will be group drops and personal drops for each player in the group.
* When playing in co-op, you kill faster and there's much better loot available.
The tip? MULTI-BOXING!
And you only need to buy a maximum of 4 copies!
With your own party of 4 and in your own game instances, you can level faster and get better loot than most of the competition. All of your characters can stack on top of each other too and they don't all require varying locations in combat. :)
Blizzard has no problem with this, it's not against the rules and is widely used in World of Warcraft and other MMORPGs.
With a powerful enough PC, you should be able to get away with this on a single PC. Each Diablo window, though, might require lower graphics settings. It will take a while for the Diablo 3 retail price to drop too, so this is something you need to decide whether to do right in the beginning or not.
There's still 4-5 months to go, so take this time to research and prepare. :)
Some links to get you started; What is Multiboxing, Dual-boxing.com Forums, MMOWNED Forums
If you guys know of any other good starter sites, let me know. I'll write more about this in the future too as this is probably what I'll be doing.