Is Windows XP the OS on that computer? If so, going from 128MB to 1GB will make a huge difference.
And, I you can get the 1.7 GHz processor for only $6, I'd go for it. However, replacing only the processor and not increasing memory will not yield much of a boost.
You're currently running two monitors off your PCI express card, right? And, you want to add a third monitor and a TV? Yes, a PCI video card should work. I think getting a PCI card with a graphics chip from the same manufacturer (ATI, or nVidia) as your PCI express card reduces the chances of any driver conflict, but I'm also sure there are many mixed (ATI, nVidia) configurations that are working fine.