The Sempron 2800+ is listed, but there is no P4 2.66 listed. There is a P4 2.60 listed, but that one should be the 2.60C that has an 800MHz FSB and HyperThreading.
Intel made a P4 2.66 that has a 533MHz FSB and no HT. The Pentium D 805 is a 2.66 GHz DUAL CORE processor. Any one of these should outperform the Sempron 2800+, especially the Pentium D when it comes to multitasking.
Intel Processor Specs: