Well, with the first board, you wouldn't have to buy new memory. It is a decent performer for the price and is very stable. It is not much of an overclocker unless you get the special bios for it. The Asus is higher, but you get no more RAM slots and you;d have to buy new RAM immediately if you are using SDRAM. The MSI has a nice setup too and it could very well be the best performer based on the chipsets. The KT266A was one of the best chipsets until the KT400 and NForce chipsets came out.
I'd probably go with the MSI, but it is a close call between it and the ECS. Both are good boards. The Asus has a decent chipset, but it is not to be confused with the NForce
2 chipset that is so popular. The other two boards have more RAM choices (the MSI 2GB of RAM and SD and DDR support while the ECS has 1GB and SD/DDR support) and more expansion slots. The chipsets are all good decent budget choices and good manufacturers. I've set up both systems and my current system runs ont he ECS that you mentioned.
So, to make a long story short: go with the MSI.
GO WINGS!