If you install the X3, you will be able to install a plain PC drive into your box. Check the lists for which drives support locking if you need to play xbox live, otherwise just about any IDE drive over 6GB will work. You will have to carfully read the tutorials out there, but once you have the chip properly installed and the BIOS flashed, you will be able to do a fresh install of everyting with AutoInstaller Deluxe or Slayer's. Do some web searching on these and you'll save a lot of grief.
You can't have a softmod in your box now if it doesn't have a hard drive in it.
Installing a modchip or doing a TSOP flash makes it much more convenient to change out hard drives, and makes you less vulnerable to a total screwup if you do something like erase or corrupt your dashboard or something else that is important on your hard drive. All you have to do is to start the box with AID and reinstall or ftp your backup, or if worse comes to worse, rebuild everything.
BTW, you don't need to include words like "important", "urgent", "desperate", etc. It tends to drive away those who want to help.