I would just upgrade the RAM and put an SSD in there and you should be good for a while. Drooping an SSD in mine was the single biggest upgrade I've done for productivity and battery life.
I have an old Dell D830 with a 1.8 Ghz Core2. I put an SSD in there and it made it feel like a whole new machine. My parents were complaining out their slow computers. I had them buy SSDs.
SSDs help in most cases, however, I once happened onto a netbook (Atom 1.6GHz, 2GB RAM) where SSDs did not really help. Slow as mollasses in Web browsing, Flash video stuttering at 480p. I removed SSD and sold this netbook immediately.
