And it's also worth noting that 840s use
IDE hard drives (aka ATA,
not SATA). A 120GB IDE drive will run about $49, slightly more expensive than comparable SATA drives.
If you do end up ordering from Dell, just give them the service tag from your laptop, and they'll match you with the best possible drive.
As buwish has already pointed out, you'll need a Windows XP installation CD for your new hard drive, or take it to a reputable computer repar company for them to install it for you. The latter will likely be more expensive though.
Also, remember to remove the battery before you replace the hard drive, and unscrew the faceplate from your old hard drive, so you can place on your new one. (visual guide: http
/www.youtube.com/watch?v=Xg_EhfPcZX4&feature=related)