I have a ~2009 MacBook. It has a swappable battery (replaced once, dead again), and it was overall easily upgradable : it received a 4GB RAM stick to replace its original 2GB (unfortunately it can't manage more than 3.5GB), an SSD to replace its hard drive, and a new DVD drive as its superdrive failed. However its core2 Duo is really too slow for the modern web (either running MacOS or Linux), it takes almost a full minute to open a youtube link. I should probably downgrade to 32 bits Linux to get back some speed, but it won't become exactly snappy anyway :)

You should be able to do a 2gig + 4gig stick combo. That's what I have in my 2008 Macbook pro, it runs 6GB just fine.