It depends on what your board supports. my PC has an MSI SlotA board that only supports 1st gen althon's up to 800mhz. I think other boards may be able to support thunderbirds (spitfire core) up to 1 ghz with a BIOS flash. Check your mobo manufacturers website.
the most cost effective chip for that board, however, is a 700mhz first gen athlon. They can be bought for between $20-$30 US (visit pricewatch.com)
I thought about buying the 700mhz chip (I have a 550) but.. its just $30 more to spend for my XE board ;-)
