Alleged hardware specifications for the Nintendo 3DS that were recently leaked online suggest the portable will utilize the same processor used in some smartphones and high-end digital music players.
IGN quotes an anonymous source in reporting that the 3DS uses two ARM11 CPUs, the same chip used in early editions of the iPhone and iPod Touch, as well as the Zune HD.
Memory-wise, the IGN report suggests the 3DS will include 4MB of dedicated video RAM, the same amount as 2000's PlayStation 2, and 64MB of general purpose RAM, the same amount as the 2007's PSP-2000.
An analysis from Eurogamer's Digital Foundry section suggests that, while these alleged specs measure up well below many modern smartphones, the 3DS will derive a performance advantage from not running a heavy smartphone OS in the background during gameplay.
Graphically, the Digital Foundry analysis suggests the 3DS' previously revealed PICA-200 chip will be able to generate roughly 10 million polygons per second and 533 million pixels per second at the IGN-reported clock speed of 133MHz.
The IGN report also suggests the 3DS will include 1.5GB of on-board flash memory, a significant increase from the 256MB found on the DSi.
Their calculation for the geometry capacity is way off. They've based it on documents for an old iteration of the Pica200 from 2006, when a later 2008 document had it performing 40M triangles per second @ 100 MHz. That would put it in the 53M triangle per second range. That's all speculation though. There's no knowing what changes they or Nintendo have made since.
Yes, it's fully backwards compatible with the DS and DSi according to the original press announcement. The 3DS cartridges have an extra tab on the side (and wider corresponding slot) such that a 3DS cartridge can't be inserted in a DS or DSi, but a DS game can be inserted in a 3DS.
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http://people.csail.mit.edu/kapu/EG_08/Mobile3D_EG08.pdf