in an ideal would every website would have a separate application pool - this allows for better isolation and means one busy site doesn't affect the others. The main drawbacks of this are

1. you may have to do updates to many e.g. if you wanted to change the .net version you have to update lots of application pools rather than just one
2. each application pool loads a copy of the .net framework version it uses and some supporting code - this is approximately 40MB of code and is loaded into RAM for easy access. This means if you have 10 sites in 1 pool, only 40MB is loaded, but if you have ten pools 400MB is needed. As ram is a scarce resource people often group a few sites into a pool, but that decision is up to you.

As to how much DNN takes, it's dependant on the version of DNN (and the needs of the site -a major contributory factor to ram usage is how much is cached so if a site only has 10 pages and 100 users it will use relatively little (<1MB), but if it has 1000pages and a million users expect much more.

In general I find a default install to take ~200MB of ram (there was a bug in earlier releases where more like 500MB was consumed (http://www.dnnsoftware.com/answers/cid/427307) , but this was fixed in 7.3.0. A site with a few dozen additional pages and a few hundred users should expect ~300MB of users - these figures are very dependant on the content though so just use them as a rough guide - it's also hard to predict when many sites share an app pool as you will not know how many sites are active (when a site unloads its app domain due to inactivity the ram consumed is freed up)

Note: regarding the spam registrations we have a solution coming in 7.3.2 due in ~1 week -in the meantime http://www.dnnsoftware.com/community-blog/cid/154984/spammer-registrations is worth a read