I have an iPhone 3g that I only use as an iPod touch. Recently it's been real slow so I unknowingly did a factory restore last night and now it's asking me to insert a sim card before I do anything (the sim card i had is long gone). I've tried jailbreaking it from this state but I've been unsuccessful.
I used redsnow to try to jailbreak but I'm thinking maybe the ipsw file I dled was the incorrect one. I've tried it with the 1,2_4.0_8A293 version and 1,2_4.2.1_8C148.

Your help would be greatly appreciated.

If you let it do a factory restore, then it would have probably restored to iOS 4.2.1 - you can confirm with TinyUmbrella - it will report the installed iOS version even when you are stuck at the "slide for emergency" screen.

You should use redsn0w 0.9.6rc8 and iPhone1,2_4.2.1_8C148_Restore.ipsw to jailbreak.