Depends what "platform" your on.
C4 requiers nowhere near the ammount of memory access comparing to Gracia Final leaked Daemons.
You can rent cheap dedicated boxes with +8gb of memory, left and right.
Any of those will be MORE than enough to run probably +2k players.
What matters is Disk I/O and Read/Write.
Getting fast disks (15k SAS disks) and lots of ram.
GF is another venture, in raw testing only i've come a bit too close to utilizing 16 gb of ram than i'd like.
I can't imagine running a Live server without atleast 24 GB, just to be sure.
L2Server will never ever utilize any processors full resources at any time, imho, so it's almost irrevelant as long as you get a Deneb/Thuban(AMD) or Nehalem(Intel) ++ in terms of microarchitecture.
I think even less will do.
Max 300 USD a month, for a server if not half that will do probably.
DDOS is another matter, unless you piss off the wrong people or your known as a dumbass, i wouldn't worry too much.
Thats without buying complete p'n'p packs from Fr3d/Kation of course.