K6's don't overclock well, trust me I've been through four of them. The best you'll probably get out of this is 420 (4x105Mhz) or maybe 440 (4x110). The blue registry screen means the CPU is causing errors.
You won't experience much difference between 400 and 450. But don't let that put you off.
It's true that many of the K6 series don't o'clock very well.
However, I've had my K6-2 350 @ 450 (retail HS/Fan, no extra cooling). It ran a little hot (55° C under load) so I dropped it back to 400. At 2.2v it runs the same temp as it does at 350!
Fact is, though, that yours may not o'clock at all. I feel I got lucky with mine. One thing you may want to try is increasing the voltage slightly (I would stay below 2.6v) and making sure that the cpu stays cool.
[This message has been edited by StuTheWise (edited 02-13-2000).]