I was able to get 390 out of mine a few years ago. Lap it and use quick silver and upgrade you fan and you'll probally run fine at 400. I was always afraid to run it over 2.5v but I have heard of those of some running fine at 2.7v but I'll bet they didn't live very long.
In the years past there were many posts sugesting these were rebadged 400's. If your looking for a real cheap upgrade they came out with the k3-450+ about a year ago. It can be had for about $60 Lots of them will run at 600 and a few at 616. Mine runs great at 600. Depending on your board to run this chip you will need a bios upgrade.