I use a DSATX which is similar to an opus 250.
It works just as a regular PC PS would, except its much smaller, and has extra things inside. startup/shutdown technology, surge protection, etc. It's great for making small custom cases.
Here's mine. it's the small board. The big one is a regular mother board.
i've never used an opus 150, but looks as if it may screw into a standard case and replace it.
I'm also not sure if the opus line is programmable like the DSATX is. but i would assume so.
as far as where the 250 would go, the same place your old one went. it's just smaller and doesnt have a fan.