I agree with ramashiva, a K6 2+ or 3+ is the way to go if you are trying to stretch the life of a Socket 7 System.
It is possible to buy K6 3+ CPU's !!!!
They have "leaked" onto the market in Australia and I am a very happy user of one. There is only 1 known source that lists on the web:
http://www.auspcmarket.com.au/
They don't take overseas credit cards so you need to be keen to put up with the hassles. They also appear on "over the counter" sales at PC fairs etc in Sydney.
My k6 3+ runs rock steady at 600 meg (although rated at 450!) without upping voltage. It will also run at 617 but my PC config is not so happy, memory , PCI issues cause one or two problems a week so I stay at 600. These occur with 112 FSB irrespective of CPU used or CPU speed, so they are PC specific. Given a better system the K63+ I'm using would be stable at 617.
Benchmarks are good, I get arround 55 on CPU mark 99 and Sandra benchmarks are comparable with similar clocked Celeron or P3's. The only weakness is memory is still low but I'm currently working on finding the optimum bios "tweaks" to maximise this. 3dmark is 3300 with a GeForce 1 DDR, which is ok for a SS7 system and makes even demanding games playable and smooth with sensible settings.
The AMDZone forum has several threads that may be of interest, look at:
http://www.amdzone.com/forum/categories.cfm?catid=13
Most K62+ and 3+ users who have posted results seem to get to 600 without many problems so either cpu is a good choice. Most but not all MotherBoards can be made to work with either a BIOS update or software tools to enable write combining and other CPU features that are normally turned on by the BIOS.
Good Luck.
Greetings from "Olympics City"