I think the best thing you can do, if possible, is upgrade the memory. 2 x 2 GB should help the system run a lot smoother and take advantage of the hypertrheading, and not cost too much. Note that computer will only be able to use about 3.2 GB of the memory - but that is still double what you have. What memory configuration do you have now? If it includes 2 sticks of 500MB, and you have 4 memory slots, you might also keep those two and just add 2 x 1GB.
By the way - are you sure hyperthreading is turned on. When you open Task Manager and look on the "Performance Tab" do you see graphs for two CPU Usage Histories? I have a Dell with P4 3GHz and found I have been using it for years with hyperthreading disabled. I only learned this after reading about a post that said some systems had shipped with it disabled. Anyway, speed on some things improved after I went into the BIOS and enabled it.