When you say jam, you are planning to record yourself playing as a band? And is your band gonna have a drummer eventually?
You are not including drums and you are not very particular about quality because you are just trying to record down ideas, don't need to be so elaborate. 2 overhead mics can do you good enough.
But if you are going for quality recording fpr 1-2 songs but doing it as a band, then it is seriously best to go to a studio and get it recorded by a pro. It's gonna cost you much less than all that equipment.
If you are instead going for recording of instrumentation one by one, layer by layer the most important things is but your system itself. Processor/RAM/HDD able to process all that data?
After all that has been considered, only then should you look into audio interface. If you gonna have a mixer, something has two line inputs is already more than enough. But then your mixer has got to have EVERYTHING you require.
Things to ask:
- how many mics do you need to use? if you are using mics on your guitar and bass amp then your mixer gotta have at least 3-4 mic inputs.
- what equipment your guitarist and bassist using? certain pedals (eg most multifx and some Digitech single pedals) have amp emulators so you can record guitar directly into the mixer without having to worry about mics. In fact they can sound better than using a mic; micing amps are just tricky business.
- some audio interface like Line 6's Toneport (HIGHLY RECOMMENDED

) even have their own built in amp emulators
- consider the fact that, if you are recording a jam with all instruments at once, whatever your guitar and basist play is gonna bleed into the vocalist's mic, so vocal quality may not be great without careful acoustic isolation.