Mithc is right, RAM alone would drag the server down. Drive size is also important, as is speed. RAID1 means two drives mirrored, slower than a single drive or striped drives, and SAS drives won't have 10,000 RPM speeds. Having SQL on the same box as the web server is also a drag, and spinning up that many databases would probably kill your RAM as well.
Child portals means a single install and a single database, though 70 can be unwieldy. Performance wise it will use less RAM, thouugh you're still likely to be swapping to disk continuously. As an example, I have a (or had, it's now almost unused...) 1.8GHz laptop with a 120 MB drive and 2 GB of RAM runningServer 2003, SQL 2000 Standard and DNN 4.3 with about 30 child portals. With very light use, 40 users or less on the entire system, response times start to noticeably drop as the system starts paging to disk. They don't get unbearable except at really peak use, maybe 100-120 users on the system, but this system does little more than serve documents in temporary setups.
I could tune this system and get better performance, but it's being replaced so it's not worth my effort. Your system should have slightly better performance in some areas, less in others, but this may give you an idea of what you're looking at. Apps will make a difference as well. Anything SQL intensive will eat processor cycles, slowing down your system's performance. Just serving static files will have little effect on RAM or CPU, so you'll do a little better.
Spend you money on RAM first. Then tune the heck out of your app, system and DNN installation. Then start looking at what else to upgrade. The IIS 6 Resource Kit has decent performance guides for tuning IIS and Server 2003, use them. You may be satisfied, or you may choose to upgrade.
Jeff