So glad Dragon Ball Heroes is selling well.
Both Taiko are going to cross the 500k mark. PS2 and DS levels are far (more than 700k and 600k respectively) but I can totally see Namco sticking with 3DS for a while, after the disappointing PSP result.
Taiko no Tatsujin sales comparison:
(Only listed those titles that sold over 600.000 units)
Week 43
[3DS] Taiko no Tatsujin: Chibi Dragon to Fushigi na Orb {2012.07.12} - 3.046 / 459.470
Week 43
[PS2] Taiko no Tatsujin: Tatacon de DODON ga DON {2002.10.24} - 10.884 / 652.879 (LTD 743.334)
Week 32 (That was the last time it charted in the weekly top 30)
[WII] Taiko no Tatsujin Wii {2007.12.11} - 3.365 / 588.273 (LTD 657.666)
Week 38 (Next time it charted was in week 50)
[NDS] Metcha! Taiko no Tatsujin DS: Nanatsu no Shima no Daibouken {2008.04.24} - 9.075 / 484.186 (LTD 691.510)
Week 43
[NDS] Taiko no Tatsujin DS: Touch de Dokodon {2007.07.26} - 3.533 / 393.923 (LTD 640.480)
As we can see, console Taiko no Tatsujin games had shorter legs than the portable ones. After week 43 neither PS2 nor Wii Taiko no Tatsujin title sold 100.000 more units until the end of its run. One of the reasons is that they got more sequels. For example, in a short span of 2 years, PS2 received 5 more Taiko no Tatsujin entries. Wii would have a yearly entry.
Anyway, it's worth noting that the second Taiko for NDS came just 9 month after the first one. And its legs weren't much affected since it would end up selling 250.000 more units once the sequel was already in the market.
Taiko no Tatsujin 3DS shouldn't struggle too much to sell another 100.000 more units, or at least close to it. So while the 600.000 unit mark is far from being a lock, it is something achievable. At least it should end up selling 550.000 units.