New Components in Old MBP?

Hey everyone,

I have a 13-inch, mid 2009 macbook pro with 4Gb RAM. This laptop has been good to me since day 1, but now that i have Yosemite installed, im having a lot of issue. It freezes and crashes and cant do anything...even when I'm running one youtube video and nothing else open it lags and freezes up. The mouse is also acting up, moving around by itself, and now it wont even click. Battery also is shot, full charge lasts me 40 minutes.

All that said, should I update and refresh it a bit, or just buy a new macbook air? Even if i do spend $300 on an SSD, new trackpad, and new battery, will it last me another few years, even with the crappy 256MB video card?

Im sure there are others out there in the current situation. Thanks for your help!