The answer is, it depends.

If you are talking about RAM (the memory in your PC), then it's 1024MB = 1GB and 1024KB=1MB, but if you're talking about a hard drive, then the problem is that hard drive manufactures a couple of years ago started rounding their numbers, leading to all sorts of confusions - they say 1000MB = 1GB, since then they can claim there 70000MB disk drive is 70GB, not 69GB.

The 1024 comes from every base number in computing being a power of 2, 2^10 = 1024