I'd actually go with Extra Biomes XL. It only requires MC Forge (Universal Version), and all you have to do is put the .zip folder from the download link for EBXL in the "mods" folder under .minecraft.
If you don't know either, to install MC Forge, open both your minecraft.jar and MC Forge.zip.
Put EVERYTHING inside MC Forge into your minecraft.jar and delete the META-INF.
Now, just put the EBXL zip file inside the "mods" folder in your .minecraft directory.
Easy as pie! For all other forge compatible mods, you usually just have to put them in the folder as well. If the mod crashes your client, just delete it from the mods folder. No more force updating!
Most Clientside mods are possible.
Sometimes do you need OP to use them. (ToManyItems as example)
And those mods wich add new blocks/items wont work unless the server has the mod installed too. The same counts for mods wich add only more recipes.