3 gigs is is enough for 6 slots, not 10. Sure you could have 10 slots, but if all 10 slots get filled you will start creating lag for the players. I assume you used worldborder to make the filled area, but you mentioned having multiple worlds likely using Mystcraft. I would suggest creating a new Mining age and deleting your old one. In order to find all types of ores, make sure to use Extreme Hills, Ocean, Plains, and Desert. Extreme Hills for sulfer and emeralds, Ocean for sapphires, Plains for Bauxite, and Desert for rubies.
If you don't trust your players enough then I suggest turning off nuclear bombs, but not the reactors. I suggest turning off mob griefing, that way creepers and withers don't damage the landscape. /mobGriefing false will do that. With low amounts of memory, I suggest turning off Descriptive books, having people making ages will use a lot of memory. Have admins make ages for various things like mining. Anything else should be fine.
As for perks for paying people, I suggest nothing like items or wealth. It will ruin the fairness and economy if restrictions are disabled for certain people. Perhaps somehow create an age with a workshop for all types of crafting, available only to VIP's. You could also offer them the ability to use the commands /spawn and /home. If your willing to get all the clients to install 2 mods into the coremods folder, you could have capes hosted via Dropbox, and you let people who pay choose a cape and skin. The awesome part of the mod is that both can be HD, so they can look nice and it won't lag other players.
http://www.minecraftforum.net/topic/1559273-146forgecoremod-hd-skins-and-cloaks-works-in-smp/
PM me if you need anymore help or ideas, this thread is going off topic and is just a talk between us at this point.
P.S. Sorry for wall of text and likely errors.