How many ircd can a 128 handle effectively?

As many as you want. However realistically it totally depends on the size of your network.

Here are some larger examples … (much larger)

EFnet

~100k users

~36 gigs per day excluding DDoS attacks and all over head (so this is actually a bit less than what the total amount should come out to be)

Freenode

~13k users

~2 gigs per day excluding DDoS attacks

Obviously you aren't going to come near those numbers, but you could easily use those numbers to get an idea of what your server will be chugging.

Additionally, the performance of your IRCd will GREATLY suffer if your linode becomes strapped for host I/O time.

Sunny Dubey