Great question, OP.
I was in a similar boat about 3 years ago, when I was considering replacing my first gen 17" MBP (with the 2.16GHz Core Duo). It was getting a bit slow, and even though I was on the current OS (10.6.8), I was thinking it was upgrade time.
But before I did that, I looked into buying either a hybrid drive or pure SSD on the advice of a tech buddy. I wound up buying a 120GB SSD on a black Friday special; it cost around $160 at the time. I installed it over the Christmas break.
I could not have been happier: it felt like a new machine. The machine felt like it had that "snappy" performance back like it was when new; in fact, it was far better than that. That computer lasted me another 2 years, eventually serving time as a desktop after the screen died and I bought an external monitor for it. I replaced it with a used 2010 iMac last year, and actually took that same SSD out of the MBP (before selling it for parts) and dropped it in the new (to me) iMac.
So, the thing for me was that the SSD solved my issue of performance, and even though SSD's aren't cheap, the cost is leaned out over the extra years of service the machine will most likely provide. That $160 I spent 3 years ago for a 120GB SSD will get you a 240-256GB SSD now, and they are faster too. So unless you need better graphics, an SSD upgrade might serve you well and be cheaper over time. I would definitely upgrade to 8GBs of RAM too: I noticed the difference bumping from 4 to 8 in the iMac, AFTER the SSD upgrade.
Finally, I went through 3 batteries in my MBP because of the bulging issue, and Apple replaced them all for free (and they were all over a year old, and I never had Apple Care on the computer either). I got them replaced at a Genius Bar every time.