Normally when you overclock the FSB of a motherboard all the buses get overclocked aswell including the AGP and PCI buses. This caused problems, esspecially with hard disks, because they became a lot less stable.
So a lot of new boards have AGP/PCI which means when your overclock the FSB the PCI and AGP busses are not overclocked aswell.
AGP/PCI Lock is the ability to lock the AGP and PCI bus speed, usually it's an option in the BIOS.
By default, most chipset fix the AGP and PCI bus frequency by deriviation of the FSB speed. Let's say you have a 200MHz FSB, the AGP default speed is 66MHz, so it's (200MHz/3). If you don't have AGP/PCI lock and you overclock your FSB to 250MHz. The resulting AGP speed will be 250MHz/3 = 83MHz, this is OUT of specs. and many Video Card will not handle this speed.
It's why chipset makers introduced the option to "lock" the AGP/PCI bus. In the BIOS you have an option to adjust AGP speed, you can usually choose from a list that looks like this :
- Automatic (the chipset will define the speed automatically, usually will try to keep the speed within specs. - 66MHz)
- Manual (the user can choose the speed by MHz from a wide range of speed)
- Some BIOS might give a ratio option 1/3, 1/4, 1/2, 2/3, etc... 1/3 is equivalent to 1/3 of the FSB speed.
AGP/PCI is mandatory for extreme overclocking, and today's Athlon 64 chipsets don't have this option working. So it's impossible to get the most of an Athlon 64 CPU because, usually the Video Cards or PCI cards will crash because their BUS speed are too high. nForce3 250Gb will correct this problem and have a working AGP/PCI lock.
You've read a lot of explainations, but what Dodger failed to mention is this: NO new AMD boards have the AGP/PCI locks. He said it was on new boards, but this feature came out around 2 years ago for Athlon XP's and Intel P4's, but VIA and nVidia were in so much of a rush to market their A64 chipsets they didn't provide functional locks on them.
What that means is that Athlon XP chipsets are better developed and better featured than A64 chipsets. It means buying a new A64 platform is a step backwards to the PIII days. nVidia is fixing this with the introduction of their new A64 chipset version, the nForce3 250.
And that's only 1 reason I complained about the early A64 chipsets.
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It depends, if you are not an overclocker and you want to upgrade now, go for it! If you really want to overclock or can wait a couple of weeks/months, new A64 chipset (nForce3 250Gb) or revised chipsets (SiS, VIA) will be interesting.
In fact, many hardcore "PC fans/gamers" who needs to upgrade these days are waiting to see the performance of the nForce3 250Gb and Intel new Socket/Chipset before upgrading.