Your biggest performance jump will come with a new video card.
Get memory and/or a new CPU without a new video card and you will see basically no performance increase as your video card is your bottleneck.
My roommate has an Athlon Classic 700, 256mb of RAM and, formerly, a GeForce1 DDR. His CPU was slower than yours and his video card was much faster. He upgraded to a GeForce3 and saw a vast improvement in his system performance. It is not, in fact, staggeringy far behind my AthlonXP 1800+, 512mb PC2100 and a GeForce4 Ti4600. His video card was his bottleneck, even though it was much faster than yours and his CPU is slower. I can't emphasize this point enough. You're in exactly the same position he was and you have the opportunity to make the same simple change for a greater boost in power.
I would recommend getting a GeForce4 Ti4200, or if you can spring for a bit more a Ti4400.
DO NOT get a GeForce4 MX of any sort. They are not true GeForce4s.
www.pricewatch.com has GeForce4 4600s listed at $295, 4400s at $218 and 4200s at $122. If you aren't already familiar with it, Pricewatch is a pricing service that links to computer shops all over the US. You can get very good deals using it.
Good luck.