I think it will depend on zire's onboard memory U can compress ur mp3 by reducing their bitrates and than mp3 can be stored n fone mem U can use softwares like Nero 6 or above and Audacity
Or u can got for Real producer basic as Zire has real player on it tho I am not sure if it has play list support ..
If ur into books u can visit
www.memoware.com for free ebooks from project gutenberg
u can get mobipocket reader from
www.mobipocket.com the standard version is free
for softwares u can visit
www.palmgear.com