I want to Frankenstein me a PC for free, NEED HELP!

[JEMC]

What you want to do is no easy task, are you sure that you want to do it? There's no guarantee that those trashed PCs will work.

Anyway, if you want to do it, here are some things that you should know about those compatibility problems.

Graphic Cards_ You shouldn't have any problems with them as long as the power supply is good enough and both the card and the motherboard aren't too old (more than 4 years).

Motherboard and CPU_ The easiest way to avoid any problems is to keep the whole package, keeping the mobo and the CPU that's in it. If you don't want to take that approach then you have to consider the "family" of that CPU and then look if the motherboard that you want can take that CPU. For example, if the CPU is an i7-920 then you have to look for a motherboard that has X58 written on it, or if it's an AMD Phenom II X4 940 then you'll have to look for a motherboard with AM2+ on it.

RAM_ Not much problem here as long as you don't start mixing them. Just look what kind of memory it is, DDR2 or DDR3, and be sure that your CPU/mobo work with that kind of RAM. For example, Intel's i3, i5 and i7 use DDR3 RAM but older CPUs like the Q6600/E6600 use DDR2. Remember also that RAM sticks go in pairs, unless it's an i7-920/930/950 in which case you will need 3 sticks.

You shouldn't have any problem with DVD's and Hard Drives as long as they are S-ATA.

In any case, remember that it's a very good idea to format any HDD you want to keep and install your copy of OS.