if what i say doesnt help check out
www.xbox-scene.com for details
1) flash the x3 bios if u havent, tutorials on how to are on this site, where to get it i cant tell u (*cough*xbins*cough*)
2) get a dash (i wouldnt recommend evox because the only advantage to that dash is the ability to easily be a cheating
[email protected]$tard.) I would go for XBMC if u want ur xbox to function as a media pc, or avalaunch if u want functionality and ease as a game backup system. (unleash x is good but u have to customize it alot and it doesnt have direct downloads of programs like avalaunch). Put it in ur C: partition using FTP (the new bios allows u to use IE to ftp also).
you get a dash where u got your bios most likely
3) make sure your dash's xbe file is named one of the startup file names on the x3 bios, (eg xbmc.xbe, avalaunch.xbe,evox.xbe)
4) see how it suits you and edit the .xml files to suit your needs.