You don't store any individual time stamp for a datalogger that runs at constant speed - you just record the start time and sample frequency and then store the samples.

You should probably look for an SPI-connected serial flash. They can be found with several MB capacity, and have quite good read and write speeds.

Unless the unit has to perform advanced calculations on each sample, it should be quite ok to use a 8051. I don't expect a data logger to try to display 2 channels at 100Hz to a LCD since we poor humans aren't fast enough to read text at that speed anyway.