What class is the internal flash memory, and how do I go about installing a larger internal card?

You can't upgrade the built-in memory, but you can add an SD (regular or high density) card to the empty slot under the battery cover.

There are two kind of "internal" flash:
- A built-in 256MB flash storage. This one is integrated in the chipset and can't be replaced. This is where all your installed applications go, as well as your Documents directory etc. (/home/user/ in the filesystem).
- A slot for a memory card, under the battery cover. This one takes SD cards as well as the mini-SD card that came with the unit (in an adapter). Buy any SD or SDHC card, install it there.

Then there's also the external slot for another SD card. It's all marked on that little leaflet that came in the box.

Dang.

If I run out of room on that card, can I install applications onto the removable ones?

You can, with some trickery, the simplest way is probably to clone the operating system to an SD card and boot from there. Search the forum, there's at least one or two threads currently active on that. (Out of the box executables wont' execute if installed on a card, the reason being that cards are mounted with the exe bit turned off.)

However, you won't run out of space on that internal flash unless you do something pretty dramatic (at least not by just installing applications. More probably by installing lots of music, pictures and other data, all of which can easily be put on an SD card without problems, unlike executable applications).
One reason the internal flash will hold out quite long is that it's using a compressed filesystem, so it'll take much more than it looks like.

Also, the SD cards can host virtual memory, which can help a bit.

It seems to be a pretty good algorithm. I keep installing stuff and it seems like I still have the same amount of free space!!

(not really, but it seems like it)

Argh! My computer can't find the tablet because it can't (or won't) install the "Nokia BB5 ADL Loader USB Generic" drivers.

Is there anywhere I can find those? The Nokia site itself doesn't host them, for some reason.

Have you tried installing the Nokia Software Update Wizard first?

I am having the same problem, but just on my laptop (after successfully flashing numerous times, it quit working out of the blue). reinstalling the wizard was no help. I have filed bug 2913 (https://bugs.maemo.org/show_bug.cgi?id=2913) but no response yet. Please vote on it.

I have also looked for the drivers with no success. The link on Nokia's site is dead.