I can only speak from my experiences with the MHS-5200A (12Mhz Model): It uses 1K-Point Maps for the arbitrary Waveforms at a samplerate of 250MS/s max. With that, the most expensive version can produce up to 25Mhz Sine-Wave. Other waveforms are available too, but beyond 3 to 4 Mhz they begin to look more and more like sine-waves because of the shortcomings in the analog part.
The 250Mhz Master-Clock for the DAC is most likely generated by an internal PLL that derives this clock from the Main Clock (There is only 1 Crystal on the PCB).
To generate several waveforms, the MHS-5200A has 5 Preinstalled Waveforms, as well as 15 save-slots that can be used to store custom waveforms uploaded via USB. These are stored inside the Lattice Mach X02 CPLD (has to be, there is no RAM/ROM attached to it).
My guess is that the 4 standard waveforms (Sine, Square, Triangle, Sawtooth, Reverse Sawtooth) are created by an algorithm configured into the CPLD-Hardware, and the arbitrary waveforms are generated by directly connecting the Memory-Output Port to the 8 Pin-Drivers that form the DAC with the R2R-Ladder attached to them.
As I said, I don't know about the design of more expensive units, but I guess they are rather similar: MCU handles User-Interface, USB-Communication, configuration of Waveforms, Frequency, DC-Offset, Amplitude, etc. And the FPGA handles the actual creation of the waveform. Either by using an R2R-Ladder or a dedicated parallel Input DAC with a high Sample-Rate. The way the FPGA gets the Waveform-Data might be different though: On a less capable unit the Waveform is most likely stored inside the FPGA, while more expensive units use DDR2/DDR3-RAM to allow huge amounts of Samplepoints to be stored.
That little board you posted looks interesting btw. Hopefully there is something similar for Altera Devices, as I'm already familiar with their Software and have the USB-Blaster cable available here.
Once I'm done with University-Stuff for this year (early October), I'll do some experiments with the MAX V I have bought a while ago.