Save it or replace it?
My mom is still using my 2009 MBP without any issue.  If the CPU is fast enough for you, I'd recommend refurbing it.  

I'd also agree with Robuust about getting an SSD.  The prices are low enough now that you should be able to get 250GB for &lt; $250 and anything smaller is still &lt; $1/GB.  If you've been getting by with a 160GB spinning drive, you'll be amazed by the speed increase.  Mine went from a 40s boot time to about 12s.

The only caveat I see is that my mom's MB DID need a new keyboard a couple years ago due to the mouse button failing to click anymore.  That was a $80 repair -- bought one from ebay and installed it myself.  So if you have any kind of HW problems, you would need to address those down the road.

Then again, you can get a complete late-model MBP on ebay for $700-ish.  If you aren't afraid of buying a used one, that may be the better way, but for what you are doing, the old one should continue to work well.