I have noticed several users having problems with their PC's recognising the new Sempron processor correctly. I had the same problem and the solution I found MIGHT be of some help. I upgraded my MB to a Gigabyte GA-7N400 Pro 2 and the Sempron 260 CPU.333Mhz At boot up the system recognised it as an XP running at 110 not a Sempron. A BIOS upgrade on the Gigabyte website noted that it was specifically for the Sempron. After downloading and flashing the Bios the same thing. However looking at the MB manual the section on setting the FSB was very slightly confusing. It stated that one switch had a seting for auto and one for 100mhz. It also stated that a 200 CPU needed setting on 100Mhz. However I failed to notice that 100Mhz was the default setting.
When I switched this to the AUTO setting this proved the answer. Next bootup the bios correctly recognised the CPU correctly as an AMD Sempron 260. I am not sure if other makes of MB have the same jumper switches for CLK_SW and CLK_RATIO but it might just be this will be similar. For the Gigabyte board it did at least solve this problem - so rec-check your MB manual and see if the Clock switch is set to the correct setting - it might just work for you as well!