How much ram can I upgrade to?

everymac.com is a great resource for this kind of info. I know off hand that your machine uses DDR3-8500(1066MHZ) memory and will certainly support up to 8gb. Though depending on what you use your machine for, and your budget, I am certain 4gb would be sufficient, seeing as your machine ships with 2gb stock. You could upgrade to 4gb for $20-25 most likely, but be sure to get the right speed, alot of whats on the market for macs is 1333MHZ through 1866MHZ. In my Macs I have ran Kingston Hyper X, G-Skill, PNY, and Samsung aftermarket ram all without any errors. When buying ram I personally choose the best price vs. warranty ratio but YMMV. I have also heard good things about crucial and OWC memory in Macs, but I haven't used them.

Good luck, if you are still using the stock 2gb it is a worthwhile upgrade. I also highly reccomend looking into purchasing an SSD for this machine. I had the Macbook 5.2 mid 2009 non unibody predecessor to this model, and the increase in speed when I installed an intel 320 80gb SSD was well worth the drop in storage space.