Just a slight correction: There would be no need to upgrade the crankset when moving to a 10 speed cassette. "10 speed" specific cranksets are slightly modified to shift better with the narrower chain but that doesn't mean you must use one. A 10 speed cassette will fit just fine on an 8/9 speed freehub. Furthermore, most 8/9 speed rear deraillers (Shimano, not sure about SRAM) will work fine for a 10 speed system. The indexing is all done by the shifter.

rhenning is correct that merely moving to more cogs won't give you any more range, just smaller steps (useful to some but not all cyclists). As the cassette range widens, smaller steps become more important to me. I'm ok at 8 speeds for a 12-25 but even a 9 speed 11-32 doesn't agree with my pedaling style on the road (offroad I'm more tolerant of the gaps).