Peter,
USB and CD versions are the same program. One is not better than the other, just how they boot is different.
If the computer has a floppy drive and will boot from CD, then the CD version should work and is very easy to get running.
Some people have had problems getting their computers to boot from USB. If yours is known to boot from USB without problems, then the USB version will not require you to use a CD or floppy drive. Check this other forum post
http://www.serverelements.com/phpBB2/viewtopic.php?t=988&sid=a5f3f78df032dccbcea5fe47f84fed9e
for a way to test if your computer will boot from USB. If you need a "kicker" floppy you will have to keep the floppy drive to boot.
Also do a search of the forums on USB booting to see what issues others have had.
USB pen drive or compact flash thru a card reader should both work equally well, or both not work
For memory, NASLite will use any extra memory for buffers, which will speed up reads/writes. 256mb will be better if you are transfering large files or having multiple computers access the NAS box at the same time. Your network speed will also have a large influence on how fast you can read/write to the NAS drives.
If you do buy NASLite and have problems, don't give up. We are all here to help one another.