Definitely worth upgrading, imo. "1.8 chip", though? Which CPU is that, exactly?
I have a P4 2.66 CPU, and I've recently upgraded from a 9600XT to a 6800GT. A massive improvement in frame rates. A 6600GT would be less of a leap, but maybe still worthwhile. Increasing your RAM to 1GB is a good idea. Loads of recent games will take advantage of it.