the 200,400,600,800,1000 is the speed of your HyperTransport and potentially what is holding you back.

normally this is referred to as LDT and is in the form of a mulitplier but your BIOS has it as values instead. think of it as this. 1000 = 200HTTx5 so it's the same as a multiplier of 5. 800 = 200HTTx4 so it's a multiplier of 4, and so on...all of this if your HTT is at the stock 200. with your HTT at 275, with this set to 1000, your asking the HyperTransport to run at 1375 (275x5)...this is not possible. even with it set to 800 (or a multiplier of 4) your aksing the HyperTransport to run at 1100MHZ (275x4) once again, not possible with the K8T800 chipset for the most part. so this means you will want to set this to 600 which is effectively a multiplier of 3 giving you 275x3=825 and well below the stock 1000MHz. this might make your system stable at this overclock and allow you to keep raising the HTT.