20 pin cable with 24 pin slot? safe?

It's safe - electrically, but probably won't work properly. Either you'll need to get an adapter to convert one of your molex plugs into a 4 pin plug, or get a new PSU with the correct 24 pin connector.

what exactly do you mean by wont work properly

I just think it meants that the MOBO won't have enough power to function as well as it should

Buy a new PSU. You need the power.

Its a pretty decent psu, just bit old, where could i buy a molex to 4 pin plug that abekl mentioned?

I just did a bit of googling and found an article saying that as long as the computer isnt using more than 355watts then a 20pin plug will be fine