It won't hurt to upgrade the memory, and its sooooo cheap right now.
I run a GF2 MX and its great for the money, if you are on a tight budget, for now I would spend the $$ on the processor.
Stay away from the 800 celeron...
I don't know how much you have to spend... but ideally I'd recommend a new mobo and an AMD processor, but seeing as you don't think u can afford that (the duron 800 is cheaper, and much faster than a celeron 800, and its clockable to 1ghz) I'd say go with a P3.
SKurj