No need for caps
Your celeron will be sufficient as long as you install what you need on it and not bog it down with random programs that eat up resource. If you're using it for hosting services, don't overload your server with too many clients/accounts, because a celeron isn't that powerful . I have an old celeron 2.4 that's hosting ~50 accounts on it and it runs just fine.
Yeah, it basically varies on the size and traffic of your clients site. But I'd leave it at around ~50 to 100 accounts max. Just watch the server load and if it gets too high... its time to upgrade or purchase a new machine.
Yeah, that willl handle a lot more. Like we said earlier, we can't give you an exact amount of clients/accounts you're going to be able to put on a server because every client/account will use different amount of resources. But a P4 2.4 will easily handle 150+