I need a new computer for 3d rendering. I currently own a 2ghz macbook. I'm a student on a budget but am willing to push the budget only if it is truly worth it. A refurb 3.06ghz iMac is going for $1000 compared to $1700 i5. Is the $700 worth it for the power/screen. Also, how does a 512mb graphics card affect 3d rendering compared to a 256mb graphics card (substantial or not noticeable)?. Thanks for the help!