I'm using an ASUS S370-DL Slotket adapter in my Abit SH6 mobo with great results. If you're dead set on going to a FCPGA PIII and your DELL will support upto 1GHz, why not try a slotket in your mobo first. At abot $8-15 it could save you alot of time and money over replacing the mobo and cpu. ASUS, Abit, IWILL, and Powerleap all make good slotkets. Be advised though, the Powerleap PL-iP3's are very expensive - $59.00(US). Good Luck.
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