It sounds like you answered your own questions. If you are not a gamer and you don't watch DVD's on your computer then my question is why would you want to uprade?

PCI-express is not really a today technology. Yes it's already here and yes computers and graphics cards are now equiped with it but it was designed to replace an upcoming bottleneck, one that we have yet to reach. For once the computer industry is out in front of the problem before it has really become a problem. In time computers will hit a graphics bus limitation and programs will suffer but that time is off in the distant future. PCI-express is supposed to help stave off that limitation by widening the bus bit path.

Of course you would see a frames per second (FPS) improvement with a new X700 but that's because it has a faster core speed and lots of memory. I currently run an X700 and it has increased my FPS but that's not because it is PCIe rather it just has a faster core processor speed and more on-board memory. In fact my old Radeon 9800 AGP could hold it's own against the X700.

I guess my answer would be leave well enough alone unless you have a secret desire to become an expert on-line gamer. I really don't believe you will see a signifcant increase in overall system speed with this upgrade. If you just want to spend money and have a cool piece of hardware to brag about then go for it. That was pretty much my motivation. ;^)