Hmm.... well, as far as general usage (email, internet, word processing) you will probably notice a slight increase in performance, but not a whole lot...
If you're adding the 128mb stick and not replacing it, then that should give you a nice boost in performance as far as how much you can do. The video ram will help performance a bit, but it's still going to suck pretty badly with only 16mb of ram... you could get a 32 or 64mb card in that machine since you've got a 600mhz cpu.
All in all, I'd say save your money and buy/build a new system..... I mean, I built the one in my sig for just under $700.