Currently I have three low traffic websites but I'm about to have a free adult website (almost entirely html and image requests) which is expected to have some peaks of 100's of concurrent users and 150k - 1 million hits a month.

The three low traffic websites contain a wordpress blog and a forum (battlesofnorghan.com/forums/ , mostly damned bot users and little active ones) along with normal html. While they are well within shared hosting limits now, they'll grow.

This month's bandwidth usage of the three combined: 7.63 GB. (24th day.)

The four websites would be hosted on a single dedicated box of course.

I've never used Linux but I'm a programmer and I'll soon need to anyway. Nevertheless obviously managed is the way to go for me, right? (I'm willing to learn and spend up to 20 hours a month managing it though.)

If my guesses are correct, something like this would suit me best (I don't know):

AMD 2000+ or better
MANAGED
512 MB DDR RAM
1 TB+ BW (the adult website will have high quality, big file size stuff)
100 mbps

VPS, I think, will be enough for you.

VPS, I think, will be enough for you.
Yes, that's true, but you'd better contact your hosting provider and talk over the details, you may start with VPS and only if the number of visitors gets larger and you will need upgrade, move to dedicated.

More importantly, what's the website address of your new site ;)

It isn't on the web yet.

VPS, while might be enough, tends to be almost as expensive if bandwidth is the same. Even if the bandwidth is one third of a dedi, a managed VPS is still costly. So please tell me what kind of a dedicated server I should go for so that I won't have to upgrade in a few months? How much RAM? How good CPU?

Because of the kind of hits you're expecting and the kind of bandwidth you will require ( 1TB is VERY VERY small for an adult site ). With 512mb ram - you will run into issues with the server's io as well. Also - your storage system is plain ide from what I can tell - for that reason, you will have issues with speed and "slowness" when the site does get a little actively used.

As you're a programmer, I'm assuming you're actually going to be busy with the development of the site and if you have buiness interest in it, the business section of this. In that case, you might be better off the do the managed hosting at the beginning to get at little headaches as possible.

Thank you!

Would 1 GB of RAM be enough then or is 2 GB recommended?

Is a CPU like AMD 2600+ Sempron still good enough?

It might be SATA2 but I doubt it is really any faster than IDE.
So SCSI or a raptor HDD is recommended then?

Depends on the volume of users you're expecting. From your data provided, I would suggest 1GB is plenty if i's plain HTML + Images. If you are smart about what you're using on the server as software it should be good. If possible consider scsi drives.