if you just want room mics you have a few options and you can get some cheapish two channel usb mic preamps, i haven't heard the quality of any of them though. mic wise if it is mostly just two of you you could get something with a figure 8 polar pattern and setup on either side of the mic so you get a reasonable amount of rejection around the sides, the other option would be a couple of cheapish large diaphragm condensors, there is an australian company called rodes that make really great mics for a low price and the sound quality is quite exceptional. i am using two on a gig i am doing at the moment in a 2000 seat venue so i can vouch for their quality. you will need to experiment with placement in your room to find where things sound best, where the least reflections come from etc. or ig you got two you could close mic a couple of the instruments but i tend to like a bit of room sound in my recordings anyway. stick away from behringer mics as they are shithouse the only good mic they make is a reference mic for rta of a room. you could always buy a couple of 58s they are great mics and you can get tons of gain and you can nail the fuck out them cos they a built like brick shithouses.
if you are using a real piano and it is an upright i would pull the front panel off if you want room mics...
http://www.zzounds.com/item--ARTTMPUSB
http://usa.rodemic.com/