Well if you want to install XP youre gonna need atleast about a 3GBHD thats really minimal depending on how much you plan to store on it thats about $45 w S&H
youll need a win98 boot disk available online at http://www.drd.dyndns.org/index2.html for free
a copy of smartdrv.exe
the imod2 kit for mounting the hd (also comes with ps2 keyboard mouse adapter, and special switched cable) all from badflash as listed above along with the v5 bios chip (that is if its a v5, id recommend this if you can find it), and i'd recommend getting the k6-2 450mhz processor while youre there
a cooling fan from tennemax.com (look for special iopener fan)
special video and sound drivers (im not sure where to find them but u can ask around)
an xp cd and of course youll need at least 64MB ram to run xp
heres a copy of a thread that may help:
1. Using a 2.5" to desktop drive adapter, with the 2.5" drive set up as master in a desktop system.
2. Booting from Win98 bootable floppy, from DOS prompt run fdisk to partition & format the drive. Reboot.
3. From DOS prompt run sys.com to make the 2.5" HD bootable.
4. Copy the WinXP CD's "i386" folder the the 2.5" HD.
5. Copy the "smartdrv.exe" file from a Win98/ME machine to the 2.5" HD.
6. If the desktop machine can boot from the 2.5" HD, then the drive can be put in the IOpener.
7. Boot the IO to DOS prompt. Type "smartdrv.exe", Enter. Then "CD\i386", Enter, "winnt.exe", Enter.
allinall it cost me about $200 but you may want to consider adding a cd-rw or something if you dont plan to have it hooked up to a network or something just so you can move files to and from...
hope that helps