To clear things up:
Yes, most digital multitracks can only record on two tracks at a time. Some on four, and some an all 8 or 16 tracks that they have. If you are going to buy one new, you'd need to give out at least $800 for an 8-track. If you want to record an more than two tracks at once, you'd probably end up needing to buy pro equipment, costing twice that.
The fostex D-108 for example is suggested retail of $1495. A 16-track, like the D160 is $2195. And, please note, these aren't portastudios. You'll need an external mixer to, which will cost you about the same, and then you also need some way of backing up the recordings, to a CDR or something...
So, maybe we should cool down a bit with all the shouting about "analog is living in the past" and "forgetting 8 tracks".

It's not that easy, because it comes at a price! 8 tracks of course are more limited than 16, nd 16 is more limited than 24, and so on.
You get what you pay for,essentially...

And yes, of course, the high end digital stuff does give you a better sound quality. But if you have been living in a four track casette world, my bet is that the rest of your equipment can't give you that sound quality anyway, so you'll need to put ot ANOTHER $2000+ bucks on microphones, preamps and effects. Well, after spending $5-7000, you then you have a nice semi-pro studio. I do get the feeling however, that this wasn't really what you were asking for...
If your on a budget, there are three ways to go:
1. Old analog stuff. Forget about the 8-track casettes. Reel-to-reels are getting cheap now, and sound much better. I payed around $300 for an 8-track Fostex A-8LR. It's old, but it works, and it can record on all 8 tracks at one time. Somebody else here got a similar Fostex 8-track that can record on 4 tracks at the same time, including a mixer for $300. You'll never get a digital that cheap. 16 tracks are more expensive, of course, but still cheaper than the digital variety.
2. Digital portastudios. They are more expensive, I think the cheapest one are the BR-8, for around $800 or so. These will not be able to record more than two tracks at once. But they will probably have lower noise than the reel-to-reels, and most of them have built in effects. They are smaller too. An analog 8-track and mixer takes loads of space.
3. Computer HD recording. Now, for this you need a computer, with a soundcard and loads of extra harddisk space (and some kind of backup-system). That's not cheap, but if you have one you can use, getting started is cheap. The number of tracks you'll get out of this is depending on the computer, and so are the amount of effects processing you can do. If you need more tracks or more simoultanous effects you'll need to upgrade the computer so that can get expensive, since you often need to buy a new computer. The sound quality will be crappy, unless you buy a high quality soundcard. If you want multiple inputs and output, that will again be expensive. But you can get started for $39 (which is what n-track costs), and you'll probably get more than 4 tracks out of a decent PC. Also: If you have loads of external effects, like reverbs and such, you can basically forget to use them when mixing. Integrating them with a computer is complicated and expensive.
That's basically it, I think!