I have initially used WPA files (format I used with my old pocket pc) but while these work fine with the built in Media player, the sound is a bit naff with extra ones such as UKMP or MediaBox Media Centre (both of these apps played the sample music files OK - which are mp3 format).
Am I better of re-ripping my CD in mp3 format or is there another format that works better on the N810?
Also what kbps is best

It's always a trade-off: ogg-vorbis has about half the size of MP3 for (roughly) the same quality, but it needs more work from the CPU, so it tends to be a tad more aggressive on battery life.

Personally, I only have a small selection of music on my Itablet for those rare occasions I find myself without a dedicated MP3 player. They're all MP3, most encoded at 128 kbps, with a few as low as 64. They play fine, although they must be an audiophile's worst nightmare, I'm sure.

Oh yeah, I use cdex to rip my CDs.

Personally I rip my CDs to ogg-vorbis (at quality level 5 or 6). They play fine in Canola, but require installing ogg-support. They may have also required some non-obvious config file change, otherwise the ogg's stopped after a song and didn't move to the next track (mentioned in another thread).

For best compatibility, mp3's would seem to be the choice. Ripping to 192kbps seems a reasonable bit rate.

Ogg's are great, but the mp3 format is more widespread.

For ripping, I agree with CDEX under windows or something like sound juicer in gnome. EasyTag is my preferred utility for editing tags under both windows and a linux computer.

Use Exact Audio Copy or dbPoweramp. EAC is freeware, as is the LAME encoder that actually does the mp3 conversion. Most audiophiles consider it the best tool for the job, though dbPoweramp and Media Monkey have their loyal supporters. MP3Tag is an excellent program for editing tags, making the playlists, adding album art etc. There's a real trade off in file size vs. audio quality - I use the "preset extreme" settings, which makes mp3's at 240K variable bit rate, so good that I can jack my Nokia into my Klipsch PC sound system, crank it up to 11, and get surprisingly good quality sound. Space required for mp3's isn't that bad, I've got just over 150 complete CD's on my N800, which occupy about 12.5 gig on my 16 gig memory card, room to spare. Beyond that, there's always mp3tunes.com, or a combination of orb server and orb's mycasting web interface to access your entire PC library of mp3's from almost anywhere on the planet.