Just to clear up, can people not 'thread crap in other peoples for sale threads, please go read the rules.
Also, xbox devkits did come in sealed boxes originally, finding one sealed is probably impossible. The best you can pretty much get is like my as new in box devkit, ive got the original box for it, original boxed matching raptor pc card and hdd, and a boxed as new debug kit also.
The 'development kit' xbox there are a couple of different simillar looking versions minus the alpha towers, they tend to run pretty much the same processor as a standard xbox, but usually run at 740mhz instead of 733. Indeed they do also have 128MB RAM. The proper development kit also has a couple of extension cards, and is in a slightly taller case, the extension cards include a USB port, serial debugging port, and a 'scsi' type connector that is used with the PC card for DVD emulation.
A good difference on all development type motherboards is the MCPX chipset is a X2 instead of a X3 like retail.
Debug kits are another story, which people sometimes also call 'devkits' which it is, though the proper development kit is much different to look at, and of course has more features. Debug kits are not always the same, they have no extension cards, tend to come in a HALO LE looking case, however most of these have 128MB RAM though some only have 64MB, all of these of course still have the X2 MCPX. Most are usually badged 'DEBUG KIT' on the front of the unit aswell, however not all are. There are also some genuine black debug kits, though they are not too common.
Hope to helps clear things up a little, i could have posted a good bit more, but theres the basics.