This is especially for Junya as she asked how I have tons of macros for my spell casts and how it will announce it in chat.
Basically, make a macro look like so:
/party Casting spellnamehere on $t
/cast Arcane Explosion Rank 6
The first line of that is the chat message, the second actually initiates the cast. You can set the message to any of the channels like /say /raid /me etc but if you want it to use the default channel you last typed in, just type the message w/o the channel prefix before it. Problem with that is if your last message was in guild chat and you forgot to set your channel back to party or raid the guild will see your message.
Also, you can just type the spell name w/o the rank and it will always cast the highest rank of that spell. The spell name has to be exactly as it is in the spellbook or it wont operate properly.
You can get all fancy with it if you want with tons of options that are all outlined here:
http://www.wowwiki.com/Most_Used_Macros
I actually modded the hell out of macros so mine wont announce to chat unless I hold down the "CTRL" key when I click the icon which was a pain in the neck to do, wouldnt reccomend it unless you can have the patience for that sort of thing.