"Best value" is a highly subjective matter - it depends what you're planning to use the PC for.
PC Advisor magazine's "Best Buy" section is usually a good place to look for the current best value machines.
Adding extra memory when you buy the computer shouldn't cost much. What do you need it for?
Transferring your data files can be done in a number of ways. The easiest is probably to burn them on to CD(s), or use a USB memory stick.