I use a 3,1 myself, and even in casual use I have noticed a great performance increase when dropping in another processor, especially for something as trivial as $70 off Ebay. Any PC NVIDIA GPU will work in OS X, though without a boot screen. Like Gav Mack said, since there is a GTX 680 Mac Edition, the EFI Rom is readily available and easy to flash should you need a boot screen. I use a GTX 660, and it's worlds better than the Radeon 2600 XT that came with my machine. I recently got another 1080p monitor, which I drove off the Radeon for a few hours, until I tried to watch a video on it, and noticed that the Radeon was offloading the load onto my CPU because it couldn't handle it.
PCIe SSDs are not expensive at all, honestly, I was able to pick up a Velocity Solo x1 for $45 and a Samsung 840 EVO SSD for another $80, and my machine boots crazy fast off of it. Threw in another 1TB and configured a Fusion Drive. I have 14GB of RAM, though running at 667Mhz, and that's plenty for me. I rarely use it all.
I do casual video editing, 720-1080p in Premiere Pro, and the CUDA acceleration in my video card helps to render faster for sure. If you don't mind me asking, what do you use your machine for? If you don't need the super horsepower of a nMP, adding another processor will give you roughly a 40% performance increase in CPU related activity.