slinki,
I had a similar thing happen to me just last night on my brand new 13 inch Macbook.... ugh! It got alternate black and colored vertical lines on the screen, then went totally black. I couldn't start it after that for sour apples. Since I had put new (4GB) memory in last week almost immediately after I bought it, I decided maybe it was a bad stick, so I removed it and put the original Mac 1GB back in - Voila! The Mac came back to life.
I also noticed that one of the new 2GB sticks had a crinkle in the paper sticker on it, so I smoothed it out and pressed it down, then re-installed the new 4GB sticks back in and made sure they were firmly slotted. Now it works just fine, so I deduce that one stick must have lost contact with the slot and stopped the Mac. Maybe yours is a loose memory stick?
Don't know if this helps, but I do wish you good luck on getting it fixed.
Noel