I was just given N810 with only the charger, how can I tell if it is WiMax edition and which miniSD cards will work with it??? Will any 3.5mm headphones work and are there any suggestions for a good but inexpensive set? Thanks

The easiest way is just by looking at it: the WiMax edition's keyboard is black, while the "plain" one's silver. Alternatively run "osso-product-info" in the terminal and look at the output (RX-48 = N810W, RX-44 = N810).

Both SD and SDHC type cards work. You should probably look into microSD as you can get better prices and/or higher capacities in that format, but be aware that some microSD to miniSD adaptors (notably Sandisk, though their cards themselves work fine) don't work in the N810.

Any standard set of headphones should be fine. Don't waste money on extras like volume buttons as the software doesn't support them anyway.

iirc, the wimax version is extra bulky to fit the wimax radio.

I'll second the micro-SD suggestion. The Kingston adapter has worked flawlessly for me, and I chose it after reading through a number of threadsx on the subject. Get one with both the micro-to-mini & micro-to-full-size adapters if you want to be able to transfer lots of files from other sources.

Just look at the label under the battery. It would say “Nokia N810 WiMAX Edition”
And there’s WiMAX MAC address(?) under the WLAN MAC address information