PCI Express 2.0 card in slot 1.0 to run dual monitors 2560 x 1440

bottlenecking or overkill??  

In short...yes.
The main difference between PCIe 1.0 and 2.0 is throughput and link speed. These really only come into play when the graphics card is unable to handle the workload on its own (i.e. requiring more memory than it has onboard) which mostly comes into play when large textures need to be loaded as in modern games and benchmarks. For basic desktop or office applications you will notice little performance differences, but if you want to play modern games at those resolutions you will want a pretty beefy card that does not need to offload any work to the system. From a value stand point it would be best to upgrade to a motherboard with PCIe 2.0, as long as all other components can be transfered, and buying a moderately priced video card, see Best Graphics Cards For The Money: May 2012.

Need more information.

Are you talking about playing video games?

If yes, are you playing on both monitors at the same time or using one for extended only?

What hardware are you using?

(CPU, graphics card, RAM, motherboard, Windows version)

PCIe 1.0 can operate at an 8x link speed, which is fine for low graphically demanding operations, presuming of course the graphics card has enough onboard memory, 128-512MB should be sufficient unless playing any game from the past 3-5 years, and should not be a bottleneck.

It's fine for anything really. You don't lose much performance even with the highest end cards - so with weaker cards you won't feel any difference at all:

Great graph, I guess I didn't realize how close it all was, although the cards listed do have a fair amount of on-board memory, I'd like to see the same graph with something like a 3850 or 9400.