"...the power of the editor nor the number of pictures to be edited is as important as the size of the files you're editing, It takes a good deal more computing power (speed, memory, and graphics capacity) to edit, say, a 30-megapixel image than, say, an 18-megapixel image."
Of course what Robert says is true but it is only a part, even a small part, of the issue. The editor is what takes the 18mp or 30mp photo and does its thing. Some are more efficient than others. PS is a hog!
"The difference can be nearer exponential than linear."
What does this mean? Well again in a real world experience you are probably talking a few seconds to perhaps a full minute.
Your decision, too long to wait?
"I was thinking of Gimp or portraitpro ..."
I have both and I have used both. Both are hogs. You will need a better machine (better than a bottom rung or budget computer) for reasonable results. My Dell is 3 years old so it isn't the latest and greatest. It is an I7 with 16GB of RAM. I have three internal 2TB hard drives. The last one of the three is my Z: drive and it is mostly given over to PS/LR and Bridge to use as it sees fit. PS and Gimpshop like and need large cache and swap files space. It also means you need fast HD's.
You are deciding between "Gimp or portraitpro" that is an odd couple? They are aimed at two totally different areas of editing. Portrait Pro is not going to be a decent Raw converter but you can add DPP4 for that. Anything PP does Gimpshop will do and it is a good Raw converter.
"... the 50D is only a 15 megapixel camera"
If you are like the rest of us, I doubt that will remain your only camera. Just sayin'. 
