The type of slot or socket that houses your processor goes a long way in determining what upgrades are available to you. Here's a look at the most popular types out there.
Socket 7, Super 7 - Socket 7 motherboards support original Pentium chips and compatible chips by AMD and Cyrix (133MHz to 266MHz), and use a 66MHz system bus. Super 7 motherboards add support for 100MHz memory and AGP, along with AMD K6 processors up to 450MHz.
Socket 370 - Standard socket for all new Intel Pentium III and Celeron CPUs. Maximum supported speed depends on motherboard and BIOS. Most Socket 370 motherboards support up to 133MHz memory.
Socket A - Standard socket for current AMD Athlons and Durons. Maximum CPU speed supported depends on motherboard and BIOS. Socket A motherboards support up to 133MHz memory.
Slot 1, Slot A - Slot 1 is used for Intel Pentium II, Pentium III, and Celeron processors. Maximum speed supported depends on motherboard and BIOS. Most Slot 1 motherboards use 100MHz memory. AMD used a similar design called Slot A for its early Athlons up to 800MHz. Slot A motherboards support up to 133MHz memory.