When I was running Cubase SX, I was running the Silverspike RoomMachine 844 plug-in in it and on a Hewlett Packard Pavilion dv8000 lap top with 2 GB of RAM and a 3.0 Ghz processor. It's not too hard to see after using it for a little while that this plug-in won't take up too much of your processing power. The system I was running it on definitely wasn't one of the best and it still did the job fine. However, it's hard to say how well a piece of software will run on a system without trying it, so I'd recommend just downloading it and seeing for yourself.
While maybe not the most realistic sounding or most in depth reverb plug-in, for being free, I've got nothing negative to say about the Silverspike RoomMachine 844. It's incredibly easy to use, and users of all types should be able to catch on quickly here. There are certainly a lot of free VST plug-ins out there, but as far as room emulators go, this is definitely one of the best that I've used. I don't know that I'd have a place for it in my current Pro Tools rig, but if you've got a VST based DAW and are looking to get a decent enough sounding reverb plug-in without spending any money, look no further than the RoomMachine 844.