I would hold off a purchase of a soundcard or mixer until you know exactly what you want and need. There will be advantages to different combinations and some stuff only works well with certain software. Know what your entire recording setup is going to be before you start buying or you may have to buy some stuff twice.
If you just need some preamps then any mixer will individual channel Inserts or Direct Outs can be hooked up to the 4 inputs on the Delta. A good compromise might be a digital mixer because they usually can use MIDI to have the mixer faders control your software. Roland, Tascam and Yamaha make some cool small digital mixers that will do it all but you would use a different soundcard that has a multi channel digital connection rather than the Delta.
Any control surface that has mic inputs/preamps will usually have a way of getting that data to the computer through USB, Firewire or it's own soundcard.
Since you didn't mention budget it's hard to be specific. Personally I would go with something like a Soundcraft M4 to go with the Delta but you will not have any MIDI control over your software. You could get one of those small USB controllers but they usually only have 2 mic preamps. Using both together would be a decent option but at that price you might as well get a good digital mixer that will do it all.
If you can throw out a dollar range than people can give you more specific recomendations. "not that much" can mean different things to different people. Prices can vary from $65 to $30,000.