There are two TMPGEnc progs one is an encoder the other is a DVD author, one encodes the other converts that to the relevant audio and video ts folders and then allows you to burn using the built in burning programme.
You are using an old Duron processor, depending on how old the motherboard is will decide if it can support Athlon XP or Sempron processors you would have to check the compatability of the motherboard, you can do that either on the makers website or on AMDs website.
If your motherboard won't support the XP or Sempron processors you will need a new mobo, which means you will have to get new memory as it is extremely difficult to find a mobo now that supports PC 133 or older memory. That is why I said processor first and memory second.
As to how much of an increase that depends on the processor and memory combination, as you can see by harleys systems he has a high spec and so his will work faster, I have and Athlon XP 2000+, 512 PC2700 memory, Asus A7V8X-MX mobo, and as I said it can take a while for my PC to encode.