The problems with recording with a computer include having enough power to last an entire concert, having the interface plus the computer to deal with, the size factor, and the general issue that an all in one recorder tends to be less of a problem and more dependable.
Three options immediately come to mind. Tascam DR60d (two XLR inputs with phantom power plus one 1/8 stereo in covering your soundboard feed. It's about $169.
For $199, you can get the Tascam DR60d MKII with supposedly better preamps.
For $299, you can get the Tascam DR70d with four XLR inputs with phantom power. Improved preamps/much smaller case/can handle larger SD cards. B & H photo was also including a free copy of Sony Soundforge Pro including some audio clean up tools from Izotope in this deal, too, recently.
Any of these will connect to your computer via USB to download files.
Buy a 5v cellphone battery of 10,000 mah or more, and you're set for power for hours.
There are threads here on each of these recorders.