I'm very much a fan of the 5400 and even moreso the 5500... There was a lot of machine in them for the money, with a decent video controller as well, and they were actually decently expandable for an AIO unit. Only real drawback was the small capacity for RAM, which stands now at 256Mb capped out.

But yes, you shouldnt have an issue with 60ns modules and generally you can mix and match a little. I have 60ns modules in my 9600 with a G3/400 upgrade in it and it runs perfectly well. The important thing is that the voltage is correct or things blow up.