Hi 4.2.1 is jailbreakable across all devices but is currently tethered on all but the 3G and old 3GS boot rom! Meaning if you turn it off, reboot it'll need to reconnected to Pc and rejailbroken! So 3GS new boot rom, iPhone 4 and ipad then don't bother untill comex or geohot releases a new one (soon)
In regards to ringtones/music get an app called tonefxs on Cydia which allows you to change most of the phone tones and download unlimtones to get ringtones!
For music get mew seek, works out at 8 dollars so like 5£ allows you to download music / app to iPod app!
You can download a few things to download torrents but unless your really clever and have a lot of time don't bother! As it's very time consuming!! I have a few sources to get unlimtones and tonefxs for free as well as unlimvideos (stream movies/series online) vwallpaper is awesome to gives you video wallpapers and video ringtones
give me a shout if you need anything
Rich