For power efficiency, I'd recommend shutting down your PC while you're not using it. If it idles 90% of the time, it's wasting energy completely. No point in being on at all. Even at idle, it will draw power, and components will wear out (not like you'd keep any part that long anyhow).
I'm not familiar with Dark Space, but I can't see why you'd need anything much more than a 9600GT for COD4, since that was a pretty decent card.
For $50, I'm not sure you can buy a better card than a 9600GT.
Oh, and to answer your question. The 4850 would give you a performance increase over your 9600GT (keep in mind, this also depends on the rest of your system, and whether your CPU is a bottleneck or not).