PCIe is forwards and backwards compatible. PCIe 3.0 motherboards just run at PCIe 2.0 with a PCIe 2.0 CPU and graphics card. Also, have you tried a USB 3.0 monitor (on a USB 3.0 port)? If you only tried USB 2.0 versions, then that explains the lag. I'm not sure if a 3.0 monitor will also lag, but it should be a helluva lot better at the very least. As for those video cards, if 512MB versions are significantly cheaper than the 1024MB versions, then I recommend them, unless you know that you need 1024MB cards.
Also, it's video cards that you're talking about (aka graphics cards), not GPUs. A GPU is a chip on a graphics card, not the entire graphics card it-self. I'm not nit-picky about a lot of things, but I am about this. Calling a graphics card a GPU is like calling a computer a CPU.
Yeah. Sorry about that, it's just that calling them a GPU can lead to some pretty confusing explanations later on, especially if someone who doesn't understand this tries to look up information after reading it referred to as a GPU.