What are you using it for?
Except for an SSD, you've already maxed out your 2008 unibody. There's no doubt that on paper, there's significant improvements with the newer machine.
BUT, what are you using it for? Browsing the web? Writing term papers? Occasional you tube? You may just want to consider using your current system until it no longer does what you need it to do. You're a student, macs are expensive, and there will always be a newer, faster cooler computer next year at the same price.
If you're having to do something more extensive and taxing on it, such as audio or video editing, CAD work, etc., you will notice an immediate performance jump with a new system.
But even if you plan on continuing to use your 08 longer, I wouldn't put any more funds into it unless it was absolutely necessary due to the law of diminishing returns. If you'd have to replace the battery and install an SSD to start using it, you're already 1/3 or more of the cost of getting a base level macbook air with your student discount.
Speaking of the macbook air, there may be another reason to wait a bit longer to get the most bang for your bucks: in 2008, the original Air was introduced and it became the first ultrabook and a status symbol for CEO's. Incredibly cool features and design, but expensive, impractical and underpowered. in 2010, the Air became the entry level notebook with drastically improved features for a far lesser price. I think the Macbook retinas are the same thing for 2012, and retina displays will be incorporated into apple's entire product line in a year or two. If that's the case, your financial prudence in continuing to wait may pay off handsomely.
Good luck with your decision.
