For most versions of Windows (all but the oldest), 128MB just isn't enough. Too much swapping to the hard drive. 384 will help everything run much smoother and faster, though rendering may not increase much in speed (if your files are small).

Be careful with the purchase however. If the motherboard is old, say, less than a 700mhz P3, it may not support 'high-density' SDRAM memory. The older motherboards recognize low-density (128mb and below), but not all understand the high-density, which are all of the 512mb chips and a good percentage of the 256. If you are running a newer machine than that, it should work.

After installing the memory and rebooting, your system should see the increase immediately. Before opening up critical files (and possibly corrupting them if the computer blue-screens), open lots of other programs and switch between them rapidly, over and over, possibly even leaving the system on for a day or so. If the new memory isn't stable in your system you may have spontaneous reboots.

Good luck, once you are running 384 you will never want to touch a 128 machine again.

Ryan