If you're only hosting one server, the celeron is more than enough.
This is very important though: is that 10mbit drop only for you, or is it shared with other customers of theirs? If it is shared, that is very,very bad because you will end up getting major lag on your servers every time that one of their other customers has a large download being hosted.
a 24 player server will run just fine on any of those machines, as long as the machine (especially the celeron) isn't doing anything else (like running websites, etc). I'd say you should even be fine running 2-3 of them on the celeron, again.. as long as it is doing nothing else.
I wouldn't use one machine for game server hosting+website hosting unless it has dual cores or dual processors
Also worth note:
- Your "quick backups" to the same machine will cause your game servers to lag badly because you'll create an I/O bottleneck
- Make sure you throttle your off-site backups so that they don't take up more than, say, 200KB/s of your bandwidth so that your game servers don't lag.