I'm planning on buying a N800 later tonight. But before I buy I'm just wondering what the sound quality is like using it as an MP3 player.

Better than most phones and similar devices, worse than my JVC stereo system.

I find the N800 sound quality quite acceptable when played through decent powered speakers or earphones. As Texrat put it, the built-in speakers are (marginally) better than the average cell phone - a comment you might call damning with faint praise. Realistically, fingernail sized speakers are always going to suck for musical content.

The Nokia supplied earbuds are tolerable, notably weak on treble response compared to my favorite Sony MDR-EX71SL set. However, they do include a microphone so hang onto them in case you get the bug for Internet phone calling where they would presumably improve on the internal components.

I have two portable battery operated speaker units that I recommend highly for mobile use. First is the Sony SRS-T77, currently out of production although it can occasionally be found on eBay. Far better than its SRS-T88 successor, it weighs under 1 pound, folds to the exact size of a CD jewel case (1" thick), and sounds way better than anything its size. For more powerful sound, I'm very pleased with the Think Outside BoomTube H2O1 ($147 at newegg), which packs a 5-hour lithium battery, 40W of clean output and surprising bass in a 3-piece, 3.2 pound unit the size of a large thermos.

I actually feel the speakers in the n800 are some of the best I have heard for a unit of it's size. Sounds better than my PSP and about the same as my laptop with stereo speakers. It has great volume for a small unit. Not to say there is not room for improvement, like an equalizer or bass boost, but that should be possibile with an media application upgrade.
It does sound good through external amplified speakers.

Also; to me the n800 sounds the same as my Creative Zen Vision W, using the same mp3 file and the same ear phones.

the n800 sound quality is pretty good. dont expect too much bass, if you are a bass kindda guy. u can pump the sound all the way up and it will not crack. after showing my buddy the n800 and they ability to play mp3. he rushed out and got one for himself.