well if all your using is some basic headphones and listening to music files... i would find a DAP that supports Rockbox (www.rockbox.org) cause that's FREE Open-Source Firmware (which is updated alot with bug fixes/performance related stuff etc etc... current stable version is v3.1 which came out Dec 23rd 2008) and does just about everything you would want from the software side of things since it supports more audio formats (.ogg/.flac/.mp3/.ape etc etc ) than stock firmware does etc.
plus it typically supports large capacity MicroSDHC cards to.
so unless you NEED 120GB of space i would opt for a flash memory based one since there's no moving parts and they would most likely use less power.(i.e. longer battery life)
me personally i prefer the Sandisk Sansa e200 (v1, v1 is important as v2's currently dont work with Rockbox) series (i have one myself) cause there cheap (not overpriced like IPOD's are ) and work with Rockbox and with the MicroSDHC slot on it you can buy larger capacity.
only issue is since the stock firmware is needed for file transfers (cause Rockbox currently cant do that) and pretty much battery charging to the players stock firmware cant transfer files to it on MicroSD cards larger than 2GB ... so you would either need a external card reader to transfer files to it... OR use the Rockbox software itself to transfer files from internal memory to the external memory slot.
but basically i would probably get the Sansa e280 (8GB) and then buy another 8GB (or whatever size you want) MicroSDHC card then you will have 16GB of space..... but depending on how picky you are with your sound... with .OGG format (which Rockbox supports) you usually can use really low bitrates (i.e. 45kbps average bit rate) while keeping sound quality close to CD (atleast on your basic headphones i dont think you will notice it to much)... cause using 45kbps average bit rate you can fit ALOT of stuff onto 8GB of storage... but in most cases (unless your really picky or have a great pair of headphones etc) 128kbps would be plenty if all your doing is just listening to it in your basic set of headphones.... and even @ 128kbps you could fit alot into 16GB of space.
another good thing about the Sansa e200 series is that it has a 'user serviceable' battery (meaning you can replace the battery yourself if it ever dies) unlike IPOD's etc.
as for your 'gapless playback' question... Rockbox does that fine
as for 'volume level's' ... i would say that my player (Sandisk Sansa e250) is definitely not bad and i would say it's 'loud enough' cause i dont think players could get noticeably louder than it for the most part.... bottom line though is it's volume level is good but everyone's opinions might vary on it since people's standards vary.
on a side note it took me a while to convert my mp3's/CD's to .OGG format and then i used a program called MP3tag (which is free) to put in the tag data so everything is organized properly in Rockbox. (i.e. keep's it professional )
p.s. in general im fairly picky on sound quality but like i say with my basic set of headphones on my Sansa e250 (v1) (2GB) player when using .ogg @ 45kbps average bit rate it's quite close to the CD itself especially considering it's using such a low bit rate where as the same 45kbps with MP3 sounds quite bad as it's 'muffled'.... typically speaking .OGG tends to shine at lower bit rates over MP3 (i.e. under 128kbps)
bottom line though.... regardless of which player you get i would advice finding one that supports Rockbox (www.rockbox.org) as you wont be disappointed with that Firmware