CPU: ARM Cortex A9 MPCore (with slightly higher clock frequencies than the A8 counterpart, due 45 nm process) instead of ARM Cortex A8 Single-core (65 nm process)

GPU: PowerVR SGX 540 (1000M pix/s, 20-35 MPolys/s), up from PowerVR SGX 530 (14 MPolys/s)

Obviously the amount of RAM will be increased as well within the course of 2 years. However, don't except this before 2011, just in time for Maemo 7.

Since Maemo 6 is targeted as a "mainstream OS", then devices equipped with this incarnation might actually have the same (more or less) hardware as the Maemo 5 equipped N900, to keep further compatibility.

I'd say it's pretty safe to get a N900 now for 2 years of usage, since I don't think we'll see much innovations until 2011. There might be a slightly enhanced device "a la N810 WiMax edition", but nothing more than that. At least that's my guess.