2.4 = 4.8 as it's a dual core.
If your old one is Hyperthreaded, it's worth about 1.15 real cpus. So it should be about 50% faster in basic cpu terms.
It's also probably got a faster memory bus, and better video card, and will knock spots off it in real application use.
1GB is o.k. for OS X but I'm sorely tempted to stick another 2GB in both my machines now the prices are down to about £20/gb. (www.macsales.com)
Just run that by me again in language that a bufoon would understand.