17 Inch Mac Desktop Screen Problem Leopard?

We bought a refurbished Mac desktop on EBay, and we had to replace the capacitors on it when it crashed. Today while the screen was just waiting (no mouse moving for couple mins), the screen just blacked out. I could hear the music still playing, but the screen was black. I turned it off, and didn't touch it. We turned it on about 3 hours later, and we let the screen just sit again. We came back a couple of mins. later and it was gry with a rick green line down the middle. I touched the mouse an the screen went black. We turned it off and now are backing all our stuff onto a external hard drive. The only thing different is we got 2 iPod touchs 2nd G 8 GB and a drawing tablet. Is the problem serious and how much money would it cost to get it fixed?