I have an MSI KT4AV motherboard and an Athlon XP 2600, both of which allegedly support 333 MHz FSB. However my computer is only running at 1.25 GHz (instead of 2.13), and in the bios i can only set the FSB from 100-280 MHz. I checked the MSI website and I do have the latest version of the bios. Any ideas?

OK, translate 100-280 to equal 200-560. If you have a 2600+ rated at 2.13Ghz, it is a Tbred that is made to run at 266Mhz (133x2). There are two other 2600+ chips designed to run at 333Mhz (166x2). Three different 2600+ cpu's. You need to positively identify your cpu.

2133MHz...133mhz...16X
2083MHz...166mhz...12.5X

I bought the processor as a combo with the motherboard. It just listed it as an Athlon XP 2600+ (2.13 GHz) 333FSB. So should i set the fsb to 133 or 166? And thanks for the help.

166 mhz

and 12.5X for the ratio?

No, just put the FSB to 166, the motherboard will take care of the rest

The XP2600+ has three versions. You can identify which one by looking at the identification number on the CPU
AMD XP2600+ AXDA2600DKV3C = 2133MHz Tbred (Model 8) 266FSB (133.3 x 16),
AMD XP2600+ AXDA2600DKV3D = 2083MHz Tbred (Model 8) 333FSB (166.6 x 12.5)
AMD XP2600+ AXDA2600DKV4E = 1917 MHz Barton (Model 10) 333FSB (166.6 x 11.5)

The last letter identifies the FSB speed:
C signifies 266 MHz FSB (Thoroughbred B Core)
D signifies 333 MHz FSB (Thoroughbred B or Barton Core)

The second last character identifies the size of the L2 Cache:
3 signifies 256KB (Thoroughbred B Core)
4 signifies 512KB (Barton core)

XP2600+ 2133 266 FSB (133*2)
CPU External Frequency = 133.3 MHz
CPU Frequency Multiplier = 16.0
CPU Internal Frequency = CPU External Frequency x CPU Frequency Multiplier
CPU Internal Frequency = 133.3 x 16.0
CPU Internal Frequency = 2133 MHz
Front Side Bus (FSB) = 2 x CPU External Frequency
Front Side Bus (FSB) = 2 x 133.3 MHz
Front Side Bus (FSB) = 266 MHz

XP2600+ 2083 333 FSB (166*2)
CPU External Frequency = 166.6 MHz
CPU Frequency Multiplier = 12.5
CPU Internal Frequency = CPU External Frequency x CPU Frequency Multiplier
CPU Internal Frequency = 166.6 x 12.5
CPU Internal Frequency = 2083 MHz
Front Side Bus (FSB) = 2 x CPU External Frequency
Front Side Bus (FSB) = 2 x 166.6 MHz
Front Side Bus (FSB) = 333 MHz

XP2600+ 1917 333 FSB (166*2)
CPU External Frequency = 166.6 MHz
CPU Frequency Multiplier = 11.5
CPU Internal Frequency = CPU External Frequency x CPU Frequency Multiplier
CPU Internal Frequency = 166.6 x 11.5
CPU Internal Frequency = 1917 MHz
Front Side Bus (FSB) = 2 x CPU External Frequency
Front Side Bus (FSB) = 2 x 166.6 MHz
Front Side Bus (FSB) = 333 MHz

It looks like you have CPU External Frequency set to 100 MHz with a multiplier of

12.5 to give 1250 MHz. Do you have the 266 or 333 MHz version of the XP2600?

If you have the 333 MHz TBred version try these settings to give 2083 MHz:
CPU External Frequency to 166
CPU Frequency Multiplier = 12.5

If you have the 333 MHz Barton version try these settings to give 1917 MHz:
CPU External Frequency to 166
CPU Frequency Multiplier = 11.5

If you have the 266 MHz version try these settings to give 2133 MHz:
CPU External Frequency to 133
CPU Frequency Multiplier = 16.0

Thanks a whole lot everybody. Got it running at 2083 MHz. You've just saved me some serious headaches. Adios