I currently have a 64GB Crucial M4 in my desktop PC.
Another 128 GB SSD should arrive sometime as well, for the desktop PC.
Also arriving soon is a laptop with a 500GB 5400RPM HDD.
I was thinking of putting the M4 in the laptop instead and take the 500GB HDD and put it in the desktop PC, to store movies and some games. Obviously game loading times won't be great, but will gameplay be affected?
Some cases have 2.5" bays, if not you need a 2.5" to 3.5" adapter. If you have either of those it will fit fine. I would say the only time gameplay would be affected is if you play fps and are the last one to spawn. Otherwise just more loading time.
Taking a laptop HD and putting it in a desktop is ok. Laptop HDs are a different form factor aka smaller. Your problem will most likely be putting the M4 (not sure of the version) into the laptop. I don't think it will fit.
I read the laptop I ordered has a 7mm HDD slot and the M5 is 9.5mm, and that you can remove the M4's enclosure and then it fits, you just need to have some fitting, small screws or attach it otherwise.
2.5 inch yes, but some laptops have 7mm slots and some 9.5mm. Like I said, the combination of the M4 and my laptop won't work out of the box, but I read that it's possible to remove the M4's enclosure to make it fit in a 7mm slot.