I responded to your welcome message before I saw this post.
If you are running WB 1.3, version DOpus 4 is definitely the way to go.
But it's a big time resource hog. If your A500 is stock, I personally wouldn't install it to launch when you boot up the machine. It'll steal a bunch of RAM and some CPU. I have an A2000 with an 030 and with plenty of RAM and it's perfectly fine.
But on my A1000's I only launch it when I need it. And in some cases I'll simply use DiskMaster. But DOpus does have more features I like to use so it just depends on the task at hand.
But if you do want to install it, you should see an Install Opus icon on your disk, and it's a pretty fantastic "wizard" where it will ask if you want to the Novice, Intermediate or Expert Install. Even if you pick Novice, it will try to modify your User Startup file. So, be aware of that.