Check you're not using lots background images, icons (especially external ones) on your toolbars, icon sets you don't need, etc. Those things can increase the memory usage quite a lot. My dopus.exe is using just under 10meg at the moment and usually seems to use between 10 and 30meg which seems pretty normal for programs these days and nothing to worry about with RAM so cheap.
It's also worth noting that Opus will (by default) hook into things like shell extensions, (right-click context menu items and extra columns in file displays), video codecs (for information about video files if you have columns like the Description, Width, Height, Duration, etc. on; also to play video files in the viewer) and some other stuff. If those things have memory leaks they'll leak inside of dopus.exe, since they're all in-process DLLs.
What makes them easier? Maybe Opus's Collections can be improved for this type of thing, or a scrap panel or area on the toolbar can be added. If we collect some ideas here I will put them together and send a request to GPSoftware so it's in their suggestion database.
Try clicking files with the middle mouse button in Opus. That will always toggle select them (unless you're in Power mode and have changed the meaning of the MMB). I like it as it means both styles of selection are always available (provided the computer has a three button mouse) and I don't have to switch modes or think about which mode I'm in.
If it doesn't work your mouse drivers probably have the middle button configured set to do a special function instead of the middle button. See here for how to configure it as the button.
FWIW, if you've Ctrl-C'd some files into the clipboard you can then Ctrl-V them into several folders. (i.e. Ctrl-V several times in different places.)
If you're using the Copy button on the toolbar, instead of Ctrl-C/Ctrl-V, then you can add the line @nodeselect to its command to stop it deselecting the files it copies, which lets you copy to one place, then change destinations and copy to another.
That's the best thing about Opus, really. You can configure it to do just about anything you want.
If you're on Vista then Opus is also the only file manager I know of (other than Explorer) which supports UAC properly.
Check out these introductions if you haven't seen them already:
pretentiousname.com/opus/index.html
pretentiousname.com/opus9/index.html
They give you a good overview of the main features and should help you decide.