Re: Need advice on AD Converter pu...
There are a lot of options out there, for any budget. Its hard to recommend just one, but here's a few ideas to get started, I've heard good things about each of these.
Apogee Duet (Rounik's fave)
Focusrite Saffire 6 USB
TC Electronic Impact Twin
Another possibility is a Mackie Onyx 8 channel mixer - it has a firewire connection but also works as a standalone mixer, and you get 8 channels to work with at once, so if you want to hook your keyboard up in stereo and add a mic as well, its all good. It gets more expensive outside of North America though so it depends where you are in the world!
There are other cheaper products out there but before you choose anything make sure that the interface is 24 bit, that the preamps are good quality and if possible, try it out before you buy it.
If you plan to record other things in the future like guitars and vocals, then make sure you try these out too - I've been stung before by an interface that sounded fine for line level instruments, but could barely pick up a loud vocal through a standard mic! 'Phantom Power' is essential if you want to use a condensor mic, and a Hi-Z input is what you'll need for guitars.
Hope thats enough to get you started, come back if you have anymore questions!