How heavy is the multi-tasknig? Is it watching a video whilst surfing or is it like having the Macromedia Suite open whilst photoshopping, listening to iTunes, surfing the web and on MSN?
If it's heavy loading on multi-tasking you're better off with a native dual core like an X2 or Core2Duo
If it's lighter use like the 1st example then an A64 will do you proud.
As for the J - ye it appears to be the addition of 64bit support