Well first you are going to need a dedicated server, you can get ones with unmetered bandwidth, such as FDCServers.net, but I would not recommend this at all. I've had personal problems with them, not just stories I've heard. The IRC servers never stay connected for 24 hours without splitting. Reason being, is they use cogent bandwidth, a very cheap provider.. and you get what you pay for.
I recommend you buying a server with xGB of bandwidth, not the unmetered ones. Usually have better speeds and quality in connectivity. Servers I know of start at $79 per month, but I would go with the $99 ones.
If you are going to host IRCd's primarily, you should get firewalled IPs. Reason for this, it can help cut down on the DDoS attacks and other related problems.
You should not get anything less than an AMD XP 1600+ or if you are going with Pentium, I would not go with anything less than a 1.4ghz, and these of course are the 'bottom of the barrel', however will do the work just fine.
You won't need a big harddrive since IRCd's take under 10mb each usually.
The ram you have in your machine can sometimes make a difference if you are hosting quite a few IRCd's, but 512mb is often enough.
The operating system that is most commonly used is FreeBSD, although RedHat will work just fine as well.
I have a few systems availible for IRC use. The prices start at $79 and range right up to $199 (which are currently in stock with instant activation). They all come with 300gb bandwidth, and we are expected to upgrade the bandwidth to 400gb or 500gb for no extra fee in the near future.
If you would like to know the specifications of any server, or would like a quote, let me know and I will be more than happy to help you out.