I'm saving up to get a wii and Pokemon battle revolution i reckon you should too
but if you like the battle frontier in diamond and you like the idea of cloning get emerald
i have never played Pokemon ranger and therefore cannot comment but i can say i heard it is fun
pokemon mystery dungeon is definitely very fun i have it myself and am still not bored of it but be careful of losing items i lost a couple of hms once had to hack to get them back
ive never played the new mystery dungeon and can't comment
pearl is ok but almost identical to diamond it would only be worth it if you have two ds (plural for ds?) to play on and trade and such
hope i helped
hazza (still drinking...
)