"data is stored in database every 5 second(256bytes)"

It will take you a very long time to fill a gigabyte at that rate!

"The pc is not reliable continuous time operation"

But only the data collection part needs to be reliable continuous time; once the data has been collected, you can upload it and do all the dataprocessing/database stuff off-line.

As I suggested earlier, and Charles elaborated, a much better system partition would be to just do the data collection on the 8051, and leave the mass-srorage and processing to a PC (or, as you don't trust PCs, a Mac or UNIX or whatever...).