Normally I'm all gung-** for the latest upgrades, and Yosemite promises to be pretty awesome, but let's stop for a second and think about this.
You have there a 6-year-old Mac. You haven't said how much RAM you have installed in it. And we already know the graphics card is going to struggle with a lot of modern requirements, like 1080p video.
For the best experience possible on such an old machine, I'd suggest upgrading to Snow Leopard and leaving it there. Once you get used to the differences in OS X, you'll undoubtedly be hankering for a new(er) machine than that one.
I'd also suggest maxing out the RAM to 6GB (1x2GB and 1x4GB, this is the maximum you can put in this machine). There's nothing you can do about the 128MB video card, and for today's usage that is definitely the weak link.
Primarily because of the weak graphics card, I'm suggesting you stick with Snow Leopard unless your needs for the machine are very modest. If you really want to invest in that machine, I'd replace the HD with a HDSSD hybrid so that it stays sprightly in performance. At that age, I wouldn't spend the money on a full SSD.
The machine on Snow Leopard and max ram should be a decent performer that will get you ready for your next Mac.