I put a 32 GB memory card into my N900. For starters - it shows 29.3GB, is this OK (namely, if I use the good olde x1024x1024etc calculation which was used for determining hard drives' space, it doesn't compute)?

Next, pretty soon after installing it, two folders were created: one is .modest, the other one looks like it is in Chinese letters and strangely ends in "st" (judging from the content, it's almost the same as the first one). They both contain the following structure: /local_folders/archive, which contains directories cur, new, tmp, all of them are empty. Is this a feature of Modest (like putting the archive to the memory card)?

And something that will probably become more important... if the card is filled up with music and photos, will it put significant burden on the system's multimedia file indexing and increase the storage of index files (I presume within the "phone memory")?

Last, but not least, is there a significant difference if you're using a Class10 or Class4 card for N900?

Reflash the phone, its most likely a chines refurbished one.
Then format the card again.

Yes, the default media player/image view does index and will use extra resorces.
Just google maemo media player wiki and look for the part that describes how to tell it what not to index.

Yes. Most, if not all "Class 10" 32GB SDHC cards only do "Class 10" speeds (10MB/s) on sequential read/writes and crap themselves out on random read/writes (booting an OS off it, for example, or swapping to it).

Keep this in mind when using SD cards.

This just makes reindexing all of it a huge pain. It won't slow down your daily activities at all.

Unless you run
in a terminal, in which case be prepared to have your phone s*** bricks for ~10-20 minutes.

Not only that, some Class 4 microSD cards are 10x faster for writes than others.

So buying a Class 4 instead of a Class 10 card was actually a good decision (the salesperson of course wanted to sell me Class 10 which was a bit more expensive).
Well, after checking the card on a computer, there was no "Chinese" folder: just one ".modest" and another "MODEST~1" folder (which are now displayed on N900 as well, at least after a reboot). Not sure if they're both supposed to be there (as far as I remember, the "~1" format would be used to represent the LFN file and folder names to be accessed in pre-LFN systems), so maybe it's a (Modest?) bug creating two separate folders.

Since no one else has yet asnwered this bit...

Yes, it sounds about right. The actual usable capacity is quite a lot less than the nominal capacity (which is always in powers of two, BTW). A lot of card's capacity is lost on internal housekepping. For example, my 8 GB card shows only 7.3 GB in any file manager. Your card is 4x as big, giving about 29.2 GB.