Gear comparisons aren't exactly easy. I've got a bunch of sites that help, but figuring out the one that works best for you means looking at them all. But there they are.
The addon PAWN does a great job but it is quite complex to set up. You have to input scale tags into it to make it work. (i.e., Math!) You can look up all the scale tags I've managed to find by using the search box at the top left with the term "scale tags." PAWN can be downloaded via Curse.
I've heard Lootzor does the same thing without having to use an addon.
http://www.lootzor.com/
WOWhead has an item comparison tool.
http://www.wowhead.com/?compare
Gearwish List lets you find the gear you should have.
http://gear-wishlist.appspot.com/
Feral druids can use the Druid Wiki's gear methodology.
http://druid.wikispaces.com/ToskksDPSGearMethod
Character Planner helps you plan the gear you want.
http://www.chardev.org/
And this site lets you see what loot your character can use, which is another nice planning tool. Click on your class icon to get started.
http://www.wow-loot.com/
I hope all this helps. Figuring this out isn't easy which is probably why I Sheep Thing's guild mate got frustrated trying to explain their reasoning why this article is better than the other one in this situation.
http://www.isheepthings.com/blog/ups-and-downs/
Browse various realms, their players and their gear, or try on various combinations of gear/talents/enhancements/buffs/gems/ to see how stats change.
http://www.warcrafter.net/
http://www.warcrafter.net/
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