I did a bit of work on this and got an SD card working (reading/writing) no problem which makes it very easy (and cheap!) to store large amounts of data. Using even a small SD card u could capture data at reasonably high sample rates for a long time. I also added a max1307 real time clock to add timestamps. I did a bit of work on an arduino shield for the circuit with all of this on it but haven't sent it off yet.
To get 8 channels I guess it needs an multiplexer
You can add an 4051 which will give you 8 channels on one analog input and use 3 digital lines.
Real Time data is required/Data visualization
This is actually easier than storing it in a storage device. Just write the results you get to serial. On the pc side read them in and graph them (processing can do this) other options could be python or VBA. There is also a few programs around to do this. A quick google found this one http://www.live-graph.org/
I'll post up my circuits and code once I have it working ok and everything is cleaned up and its not too embarrassing 