I am trying to decide between getting an Ibanez sz320 at the guitar store(about 450), and a Jackson dk2 off ebay(about 500). The Jackson is $600 at the guitar store. Should I just go ahead and buy the jackson off ebay? I have tried both and liked the jacksons feel just a tiny bit more, due to its longer neck
(I'm used to longer scale necks).

So, should I just get the sz and spend the rest of my money(around $400) on an amp (peavey valveking)....
Or should I hold off on an amp and buy the Jackson??

If you prefer the feel of the Jackson then buy that; the playing sensation is, without equal, the most important thing when buying a guitar; tone can be changed in any number of ways but the feeling of playing a certain guitar can't be changed anywhere near as easily.
R.I.P. My Signature. Lost to us in the great Signature Massacre of 2014.

Quote by Master Foo
“A man who mistakes secrets for knowledge is like a man who, seeking light, hugs a candle so closely that he smothers it and burns his hand.”

I'm sure a bunch of people will rush in and tell you to buy an amp, but honestly, its no fun playing a guitar that isn't as good as another one. I say go ahead and buy the Jackson.