How many MB are in a GB on a PC

It varies on the units, depending on whether or not you're talking about SI. Its more commonly recognized that there are 1000 MB in every GB, but your computer should tell you that there are 1024 MB in a GB, because computer memory rises in increments of x^2 (Powers of 2) Ie. 2, 4, 8, 16, 32, 64, 128, 256, 521, 1024.
The quick answer? Either 1000, or 1024 ^^