Hi!
The problem on an old rig is often that you reach a point where you need a new CPU, and then everything really needs to be switched as you need a new motherboard, which uses a different type of memory etc...
So, for an old rig like that buying a new low-mid card like a GTX560 ti may create an unbalanced system that will still be rather slow anyway... So I would either buy a second hand GTX460 for your old rig on ebay for a few bucks that would for sure boost your performance in many cases (if you have a PSU that can handle it), or go for a new basic gaming rig with a cheap Z77 motherboard, i5 CPU, 8gb memory, 750w PSU and a GTX560 ti...
The problem with going for a second hand GTX460 or similar in your old rig is that it will run most games accept CloD
Edit: I bought a new rig (see sig) last year to replace my old Core 2 Duo 3.0 Ghz with 8gb memory and a GTX275 GPU. Just for fun I put the new GTX580 GPU in my old rig for a day... Most games got a boost, but CloD only gained 1 fps
With the 580 in the new rig it doubled the fps (from an average of 35 to 70 in Black Death if I remember it correctly). Same memory amount and OS in both the old and the new rig. CloD very much needs a fast CPU to fly it seems...