I'm running the reFX Claw software right now on my system at home without any performance issues at all. I usually run it in Logic or rewired into Pro Tools. The computer that I'm running it on is a Mac Book Pro consisting of a 2.2 Ghz Intel Core 2 Duo processor and 4 GB of RAM. My audio interface is a Digi 002R and I monitor with a set of Adam A7's. I can't say that I've had any problems whatsoever running this soft synth within the aforementioned rig. If you're unsure about how well this will run on your system, I'd encourage you to try it out being that it's free.
If you're interested in free software, and specifically software synths, I'd definitely recommend looking at the reFX Claw. It's not a ground breaking piece of software by any means, but it can be useful and it's free so there's really no excuse for not at least trying it out. The sounds definitely aren't the most realistic, but they're useful nonetheless if used in the right situations. It's a freebee so if you're reading this you should stop and try it for yourself!