I've built several K6's. My experience dictates the FIC 503+.
K6III350 @ 450MHz 4.5*100
K6III+450 @ 600MHz 6*100
K6III+450 @ 616MHz 5.5*112
All of them still up and running after many hours of use. Less than $80.00.
On this board you can expect to carry most of your peripherals on to a new system when ready, AGP, 3 PCI's, support for DIMMS, as well as the hard drive, CD ROM, FLOPPY etc.
A word of caution though. As the DURONS / T-BIRDS keep dropping in price, the K6's are rising, watch for the point of diminishing return! It's real easy to get carried away with the spending once you get going.