For a comparison, here's the VGC data for some other systems in USA.
3DS | PsP | X360 | Wii | Ps3 | DS | |
Week 1 | 419,000 | 408,000 | 305,000 | 463,000 | 139,000 | 486,000 |
Week 2 | 94,000 | 170,000 | 82,000 | 153,000 | 57,000 | 171,000 |
Week 3 | 70,000 | 111,000 | 43,000 | 111,000 | 89,000 | 167,000 |
Total: | 583,000 | 689,000 | 430,000 | 727,000 | 285,000 | 825,000 |
500,000 seems about reasonable. It won't have the same impact as the PsP did, and it's obviously no Wii or DS. In between 3DS and X360 definitely does seem the likely spot.