Yo,
Assuming all other software and hardware doesn't change, essentially you're going from a 1.2 Ghz CPU to a 1.53 Ghz CPU, and also from 133 Mhz RAM to 266 Mhz RAM.
CPU, you're talking about a 330 Mhz increase, so on the order of a 25% theoretical increase in calculation capability.
As to SDR vs DDR, most applications show not more than 5-10% bump in performance.
Realistically, you're talking about a maximum peak performance increase of 30-35%. But depending upon what application you are trying to run, the CPU or RAM may not be the bottleneck (eg, the video card). Note that since your HDD isn't changing, boot and load times should be virtually unaltered.
One note: since you ar upping from 256MB to 384MB of RAM, you will likely see a couple things that will operate significantly faster than the aforementioned 30-35%bump, especially under W2K and WinXP.
-PCM