The ram used for Cache in PC's can be rather fast. 15nS is about typical. If you are sampling at 100M/sec
you may need faster ram than 10nS by the time you take into account delays,setup time and circuit capacitance.

Rather than a CPLD, you might wish to investigate using FPGA's and having the cpu upload the FPGA config at startup. I did a project where I could upload the FPGA config via the serial port which made for fast debugging. You could also load specific trigger logic (or whatever) into the FPGA. If the cpu has enough flash and is programmable by the application, you can have a PC program upload new features into the cpu flash which then loads the FPGA. I would suggest something like the Spartan series from Xilinx.

To get extra speed from your memory system, you could use two 8bit wide chips and run them in 'ping-pong' configuration (odd and even) or use a 16bit device and have the fpga/cpld accumulate two bytes to write.

The digital side of the oscilloscope is reasonably straigtforward - the analog side would give me grief. Just the other day I was contemplating such a product and pondering about using Windows CE device and how to get the data into it. Most of the cheaper palmtop CE devices do not have a serial port these days and IRDA is falling out of fashion - Bluetooth seems to be the go but a RS232-> bluetooth adapter is a bit pricey (>USD$100) in small quantities. When I can do RS232 -> bluetooth for about USD$30 in small quantities, then I might start using it.

I did a datalogging system for my racing kart recently and I looked into using Palm or CE based palmtops. I gave up using my Palmpilot as I could not get enough control of the o/s to do the serial comms at the speed I required. I ended up using a HP IPAQ2210 which has compact flash and an inbuilt serial port. Speed and comms were not a problem! Windows CE was real-time enough to do my task - and I could set task priorities etc. The learning curve was pretty steep (even for the Palm) especially if you're doing special graphics. The free CE tools from Microsoft were really good - I used VC++. The doc can be a little patchy - so give yourself plenty of time!