Here's some advice- 2009 mbp is fine. Are you talking about a 13, 15, or 17? the 13's are still core 2 duo- not awful but I think battery life is shorter. I wouldn't buy a MBP13 compared to a MBA13. or even an MBA11, provided you have an external monitor to use at home where you do most of your work. a 15 or 17 start off with an i5 dual core and the option is an i7 quad core. my previous work MBP had a i7 quad before and it is plenty fast (I made these 2 changes too)
You're going to want to make 2 changes:
1. upgrade to 8GB of RAM. Should cost about $40 or less. I think the 2009 unibody macs already use DDR3. if not, I would pass as DDR2 RAM is getting expensive.
2. upgrade to an SSD. Take the existing HDD and put it in an enclosure for media storage, or better yet, for Time Machine backups. 256GB if you can afford. Agility 4 SSD's are often in the $150 range for 256GB. It's not the absolute fastest but it is worlds better than a HDD.
actually, no matter what option you choose, I'd set money aside for these 2 upgrades. unibody macs are very easy to open up. with my older MBP, I spent $29 at Microcenter for 8GB of DDR3 1333 and $99 for a 128GB SSD. I use it for work so my storage needs are not that much. I used the 500GB drive that it came with for media storage, mp3s and movies and stuff, and I kept a 320GB ext HDD for Time Machine backups.
edit: just took a look- I guess the very first unibodies carried a c2d for the 15s. I would see if a 2010 with an i5 is available in your price range. That said, the 2 upgrades will still make a 2009 or 2010 MBP more than viable for school.