Most bang for the buck on mobo's for overclockability, i'd say the EPOX 8KHA+, it goes for well under $150.
For RAM, get either Crucial or Corsair (crucial should be much cheaper), you should be able to get over a 50mhz total overclock. You should be able to get the FSB up to at least 145 from the default 133 setting. The multiplier on the XP1800+ is 11.5 i believe, so this gives you 145*11.5=1667.5mhz, a good 133mhz overclock from the default 1533mhz