I heard that your motherboard is a factor when considering OCing.
Is this true?
If so, is this board ASRock Z68 PRO3 GEN3 ATX LGA1155 Motherboard
a good motherboard to overclock an intel 15 2500k to about 4.5ish GHz?
First of all some motherboards don't allow overclocking. The one you mentioned does allow overclock so no worries there. Next there are some technical stuff that could limit your overclocking. You could look up the results other people have obtained with that motherboard to have some idea about what you could expect.
Also remember that every component is different. You could have everything set exactly as another person and get a better or worse overclock