Trader
Your machine is very similar to mine. I went with a Celeron 800 for my last upgrade. These Celerons have a 100Mhz FSB and they overclock like a mad dog. You should easily be able to get 133+Mhz FSB. Mine will do 143 but this is not stable, I run it at 138. You already have PC133 RAM and going to 133+ will really open it up. Your video card should also handle the over clock, my ATI 128 AIW did. This is the best upgrade you can do for $50.
You could also try a Pee3 1.0GHz 100FSB. I have seen claims that people have gotten this to run at 133Mhz FSB. But I don't think this is as sure a thing as the Celeron. If you go this way you need to get a certain S-code. Do a search, I think this has been discussed here. One drawback, this chip is over $100. Either way you have one more upgrade left in that board. Good luck.
I too am looking a the Northwood for my next major upgrade.