|"I recently upgraded my old sempron cpu to an Athlon XP 2700+"|
In case you weren't aware, the socket A Sempron IS an Athlon XP. AMD changed the name of the AXP to Sempron after they released the Athlon 64. What was the model number of your "old sempron"?
Despite being advertised as a budget line of AMD processors, Sempron socket A processors are simply re-branded Athlon XP processors
All socket A Semprons (except one) run at 166MHz frequency (333MHz FSB). Your AXP 2700+ also runs at 166MHz frequency. So if your Sempron was running at the proper frequency & you haven't touched the settings, the 2700+ should also run at the proper frequency.
Try running CPUZ to check that your CPU clock settings are correct:
"the processor hasn't increased performance in my pc"
You didn't say what speed the "old sempron" ran at, but if you went from say, 2.0GHz to 2.16GHz, I wouldn't expect you'd see much of a difference in performance, if anything.