MemTest is pretty good at showing failing/bad RAM modules. It can be a bit of a task identifying which is causing the problem though.
As to replacement, you should use matched pairs of modules - ie the same size and speed - but the manufacturer matters little although it is always best to use a good supplier such as Crucial or Kingston. You would also be wise to match the speed any new RAM to that existing in the PC as all RAM will operate at the speed of the slowest installed. However different sizes of RAM in different pairs (say, one pair of 1GB each and another of 2GB each) is fine.
Crucial provide a scanner on their site which will examine your system and advise what you can install.