Oh man, there are tons of options out there! And you will enter a world of constant buying - so beware (check out the evil spiral at http://www.authenticaudio.com) ....
My favorites for the lower end stuff that really can grow as your recording skills grow - 2-4 inputs at a time are:
1. Presonus Firebox - includes Cubase LE - good quality, firewire port needed and has problems with windows media edition installed, but is probably the best value out there - $299 ($240 for a blem one at musicians friend). Some free plugins before May 31 on the full price ones.
2. M-audio - FW410 (also firewire) - $299 includes Ableton Live, not as nice as Cubase, but if you want to go to Pro Tools someday, you can buy PT M-powered. There is also the FastTrack for $199, but I doubt it is as good.
3. MBox 2 - $450 but includes Pro Tools, pretty good interface - USB compatible
Mackie has some stuff - upgrades for their mixers (if you need a mixer anyway) that includes Traktion software.
There are EMU cards available that seem pretty nice.
I just don't know anything about the Line6 recording stuff and there are a bunch of USB keyboard controllers that have built in audio interfaces in the $250+ range.
Get an SM57 and mic it up - that will probably sound the best anyway.
Heads Up - if you have a local guitar center, they have their standard "holiday" blow out sale coming up - Memorial day and Labor Day - big cuts on some clearance and used stuff, and if you know what you want to buy new, they are more able to work you a deal.
Please feel free to PM or email me if you have any questions - recording is my real hobby... :lol: