so if i added two 512mb at 133, i would gain some speed?
You only need to replace one stick as you already have a 512mb stick installed. Pull the 512 stick & see if it has a label on it that shows either PC133 or PC100. Purchase a stick that matches it speed wise. Install it, replacing the 128mb stick. It would also be helpful If you can acquire a stick of the same brand. You then wouldn't have to concern your self with compatibility. Some of those older machines were pretty fussy when it came to certain brands of RAM. Some brands worked, others wouldn't. You can mix PC100 & PC133 RAM but the machine will default to the lower speed RAM module you have installed.
Realistically you are only going to gain 384 MB RAM over the 640 installed. As pointed out any speed/performance increase would be negligible. Also keep in mind, unless you have a dedicated video card installed, that a small portion of the RAM will be allocated to video memory.
Sources for RAM would be Newegg [new], Amazon, eBay, local electronics recyclers etc. Purchasing from a local recycler would allow you a quick exchange if the RAM wasn't compatible. I wouldn't be surprised if you could get it for free. Pay attention if you order RAM. You want Desktop not Laptop RAM. Google PC133 RAM.
Identify RAM: http://www.simmtester.com/page/news/showpubnews.asp?num=45