MX400 cores will ussually do 250Mhz with good cooling but the small performance boost you get from it isnt worth the trouble. Overclocking the memory is much more rewarding.

215 mem is pretty good for the average MX400. My ABit Siluro does around 215. I think i damaged it actually from overclocking too high.(been up to 270/230 with artifacts)
Sometimes on win98 systems it messes up the colour on the start bar. otherwise its still ok.