Does anyone know if N800 can use the 4-bit or 8-bit transfer mode? If it can't then there is no benefit of getting 4-bit or 8-bit card... My guess is that the data bus is only 1-bit.

It is very likely (99%) that 4-bit interface is there and is used for SD cards. I don't think 8 bit mode is there and all 13 MMCmobile pins are present in the connector. This is very new standard and the connector is more complex (2 rows) and it in not very useful right now as cards are not fast enough even to saturate the 4 bit bus. I'm also not sure even 4 bits are used for MMCmobile/plus cards as this is quite new feature in linux (similar to SDHC). I guess best is to use SD cards with stock n800 firmware right now.

I concur with fanoush, it's probably a 4-bit SD interface in there. The 1-bit interface is typically the I/F that's integrated into some of the XSCALE cpus. In my Tungsten T3 the CPU has this interface, but there's an extra SD interface chip too that is used instead, this one provides the 4-bit interface. IIRC the Dell Axim 30x (also an XSCALE device) uses the built-in 1-bit CPU interface, but don't take my word for it - I haven't looked at the Axim HW myself. I have looked at the T3, and it has a dedicated SD interface chip. If anyone of you are willing to look inside the Nokia you may find out things.. I'm not going to, hopefully my N800 arrives tomorrow (unless they sent it by truck for the last 2000km, which I suspect at this stage.. grr!) and I won't void the warranty :-)
I googled a bit and found this page: http://mmc.drzeus.cx/wiki/Controllers
The chip in the T3 is the W86L488 SD/SDIO Winbond chip.

You can buy MMC Plus cards many places now, and they promise to be twice as fast as SD, but I haven't seen many devices that support them (in Plus mode) yet. Some of the high-end cameras, possibly. My pocket camera handles it as just another SD, apparently.

Linux Kernel patches adding SDHC support were submitted fairly recently - hopefully they'll be incorporated into a N800 firmware release at some point in the future.

Has anyone tried the 150x 4Gb Transcend cards?

Edit: nevermind, I just noticed they work

Hmmm...

As apparently 8GB SDHC cards will not work, and after some soul-searching, I decided to hold out on buying a N800, at least until (and if?) a kernel with support for SDHC becomes available.

Besides, there are enough other things still wrong with the N800 to warrant cauteousness.

OK... In other words the N800 should have up to 4x faster file data transfer rate with SD than N770 with MMC cards? (MMC cards had only 1-bit interface?). This is good news since the speed, especially write speed, to the MMC card on N770 was ridiculously slow... I bought external USB reader/writer and it seem to be like 3-5x faster when writing data to MMC.

Has anyone tested how long it takes to save a big file into the N800 SD card to calculate actual write speed?

1-bit for MMC applies only to first generation of MMCs. 770 came with MMCmobile, which I think is latest generation and 4 or 8-bit. Thought I don't know whether or not 770 used those bits.

I only see 9 (standard SD).

Update - 4GB Transcend 150x does *not* work. I don't know why people post that they have it working when it does not. Try copy a 100+Mb file to the card and watch it fail. 90 bucks down the tubes.