What everyone seems to forget is that a PCI-Ex 2.0 x8 is essentially a PCI-Ex 1.0 x16, so the difference isn't going to be monstrous; it is noticeable though.
Thats true in part but PCI-E 2.0 is more than double the bandwidth of 1.0, unless you meant 1.1 which is 2.5Gbit. The thing is, as said in the Op, there is little difference but as the resolution, and more importantly, the "power" of the card tested increases then the gap becomes far more noticeable, for example, the first graphics card to exceed AGP 8x specification was the 8800GTX/Ultra, however, PCI-E 1.0 was available 1-2 years before those cards were on the shelves (my guesstimate there).
The new top dog....the HD4870x2 is pushing greater than the 2.5Gbit 1.1 spec bandwidth so the differences in performance between 1.1 and 2.0 OR 8x and 16x will be more prominent...........if that makes any sense?
Here is an extract from the Wiki.....................
In PCIe 1.1 (currently the most common version) each lane sends information at a rate of 250 MB/s (250 million bytes per second) in each direction.
PCIe 2.0 doubles this data rate, introduced in late 2007, PCIe 2.0 is found on newer systems such as those based around the Intel X38 or AMD 780G chipsets.
The latest proposed PCIe 3.0 standard will increase the speed of the links further (tentatively scheduled for release around 2010).[1]
Each PCIe slot carries either one, two, four, eight, sixteen or thirty-two lanes of data between the motherboard and the addin card. Lane counts are written with an "x" prefix e.g. x1 for a single-lane card and x16 for a sixteen-lane card. Thirty-two lanes of 250 MB/s (PCIe 1.1) gives a maximum transfer rate of 8 GB/s (250 MB/s x 32, i.e., 8 billion bytes per second) in each direction. However the largest size in common use for PCIe 1.1 is x16, giving a transfer rate of 4 GB/s (250 MB/s x 16) in each direction. Putting this into perspective, a single lane for PCIe 1.1 has nearly twice the data rate of normal PCI, a four-lane slot has a transfer rate comparable to the fastest version of the old parallel PCI-X 1.0, and an eight-lane slot has a transfer rate comparable to the fastest version of AGP. However the data rates cited must be derated because 8b/10b encoding is used in the physical layer. The link transfer speeds cited are to be considered maximum theoretical data rates.
PCIe slots come in a variety of physically different sizes referred to by the maximum lane count they support, ie. x1, x2, x4, x8, x16 and x32. A PCIe card will fit into a slot of its size or bigger, but not into a smaller PCIe slot.