qa1, the numbers I pulled were from VGChartz, which are not 100% reliable, do cover global numbers.
For another source, let's take a look at
Wikipedia.
MAIN ENTIRES:
Red, Blue, and Green (20.68 million)
Gold and Silver (15.2 million)
Ruby and Sapphire (13 million)
Diamond and Pearl (16.81 million)
Going by these numbers, D/P even surpassed G/S in lifetime sales.
"THIRD" VERSION:
Yellow (8.86 million)
Crystal (3.85 million)
Emerald (6.32 million)
Platinum (6.39 million)
Again, the fourth gen passes both second and third (looks like Crystal was a "failure" compared to the other entries... still, many games dream of just making 1M in sales)
REMAKES:
FireRed and LeafGreen (11.82 million)
HeartGold and SoulSilver (3.74 million Japan-only)
Not sure what the breakdown on FR/LG is, but we'll see how HG/SS does once released outside of Japan (which is pretty soon, huh?).
While it may not be a massive upward trend, it does show that the Pokémon brand is as strong as ever. Would Nintendo want to sell more? Who wouldn't? But they're raking in the dough as it is. Most publishers wish they could have a game 1/4 as successful as Pokémon.
And, hey, it's fun to talk about other things when Pokémore isn't releasing new works. :P