Sonar will work nicely with the 740. There is a PSR740 instrument definition files available for download for the software. You can dio all the editing in it's sequencer and then record audio tracks in it, adding effects as you go. The 2 should work well together.
I cannot compare Sonar to Sound Forge because I've never tried it ( S F ) , but I'm told that Sonar is a good bit easier to learn and navigate. I came to Sonar from Cakewalk Express, which is a very scaled down but very useful Cakewalk software, so I found Sonar to be easy to navigate, as the OS and interface look and work pretty much the same. Sonar just has a lot more features.
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AJ