So I was just wondering, how much of a real performance increase would you get from upgrading from a PCIE X16 slot to a X16 2.0? Is it worth buying a whole new motherboard or is the change almost unnoticeable?
I wouldn't tell that the upgrade from the PCIe revision will do much for the card, but the whole subsystem that works with that bandwidth.
If you want a plain answer, then: no you won't notice a whole new world given the specs. The "complex" answer is that if you change, for example, from HT1.0 to HT3.0 would be a huge upgrade from the whole system standpoint so you will notice a real benefit, but won't be because of going from v1 to v2 of the PCIe.
The PCIe 2.0 standard doubles the per-lane throughput from the PCIe 1.0 standard's 250 MB/s to 500 MB/s. This means a 32-lane PCI connector (x32) can support throughput up to 16 GB/s aggregate. The PCIe 2.0 standard uses a base clock speed of 5 GHz, while the first version operates at 2.5 GHz.
That depends on a couple of things such as the rest of your platform and what games you are playing. Just Google articles on PCI-E scaling and you will see a couple of different scenarios. If you really want to know more than read a few of them, but the summary is the difference just depends and can be around 15% or less than 1% depending on what's tested. Overall the difference is small and not worth buying a new board only for that. Unless you also want to upgrade to SLI or Crossfire and your current board can't do that you may be better off spending money elsewhere on your system or just saving for a complete redo.
Also consider that with some of the LGA775 comparisons, the PCI-E 1.0/1.1 setups use older core logic and thus older memory controllers compared to say an X38 board with better timings (hey some reviews don't even bother to manually set the timings) which automatically gives a performance increase regardless of the GPU. Even if the timings were the same the X38 or P45 board has a small advantage simply due to improved memory controller performance.