whats the difference between a PCIe 2.0 x16 slot that runs on x16 and a PCIe 2.0 x16 that runs on x4?also whats the difference between PCIe and PCIe 2.0?what happens if i place my HD 4890 that support PCIe 2.0 into a PCIe 1.0 slot (not PCIe 2.0)?and also whats the difference if i place this card once in a PCIe2.0 slot x16 running at x16 and once in a PCIe2.0 slot x16 running at x4?
The difference is in the bandwidth. x16 constitues the number of lanes, with 2.0 the bandwidth is 500MB/S per lane, with 1.x it is 250MB/S.
The difference between PCI-E x16 2.0 and 1.x should be neglectible for that card, however putting it in a slot running in x4 will most likely result in a performance loss. How much you would loose depends on the rest of your computer.
Just remember that x1, x2, x4, x8, and x16 slots are all physically different, but due to the nature of your question I think you are already aware of that.