You've got a BX motherboard, so you're in good shape.
Go pick up one or two 256 MB PC133 DIMMS, you shouldn't have to pay more than $45 a stick even for name-brand memory.
For CPU's, you have a lot of choices. The board supports a 133 FSB, so any flipchip P3 or Celeron will work fine, as long as you also pick up a FCPGA slocket.
Viper GTS