> What if there are are multiple CPU components?

Usually, multiple CPU components mean multiple identical CPU cores at same clock speed, so this is spec'd in the form of "2x 800 MHz", which in this context doesn't equal to "1x 1600 MHz" of course.

> Cherrypal says "MHz" here and I dunno if Cyrix did that

They didn't.

http://en.wikipedia.org/wiki/Performance_Rating#Branding
http://en.wikipedia.org/wiki/Cyrix#PR_system

> How do they describe the speed of these machines with multiple cores now?

See above.

> What do other SoC-system sellers do?

THTF/LimePC for instance add up the DMIPS figures of the MPC5121e's components:

http://www.google.com/search?q=site:limepc.com+1700mips

The MPC5121e's e300 core alone delivers about 800 DMIPS at 400 MHz.
In my opinion, running an industry standard benchmark on either component and adding up the results is a slightly more truthful way than just adding up the clock frequencies, especially with asymmetric designs like SoCs, but still far from ideal.

> Buyers are supposed to pay attention to what is actually said.

Cherrypal's processor spec for the C515 is ambiguos at best.

> It'd be sneaky if the SoC had poorer performance than
> an 800 MHz CPU, but I don't know if that is the case.

Actually, if the C515's SoC's ARM11 core operates at 500 MHz it *has* poorer performance compared to an ARM11 core operating at 800 MHz as well as compared to devices using the same SoC but with the CPU core operating at 800 MHz. The CPU core's general functionality is always usable to its full extend on the consumer's part whereas the usage of other cores like DSPs depends on specific software support for them. Most software won't be able to use the DSP so I'd call simply adding its clock speed to the CPU core's clock speed sneaky indeed.

> its movie-playing ability, which I read was good.

...and would be even better with a faster clocked CPU core.