Interfaces usually come in 2 or 8 channel Analog to digital and digital to analog blocks. Sometimes 4, which is a very useful number for stereo recording (you always need more than you think and it leaves options open).
At the low end of the range there isn't that much difference between similiarly priced units - certainly not enough to make a fuss over. All the convertor chips are made by a couple of companies - it is the analog surrounding it that makes a difference in sound, and price, not the chips so much.
You have the price you can spend, now consider the connection format. USB is probably the best bet today - FW isn't a good option for laptops since few have it and adding it is a questionable proposition. A shame, since the best sounding (not just me but SOS, too) interfaces in their price range is the TC Konnekt line. I use a TCK 48, but they are all FW.
Roland is a good choice since it (theoritically) should have the least problems w/ sonar and they should be able to help support their own software ;-) A lot of people use Focusrite. Do a search, there are plenty of threads and suggestions. The most important thing is to be able to return the unit if it doesn't work w/ your system and exchange it for choice #2. Most interfaces work fine, but there is always the off chance you'll spend the next month doing tech support rather than making music, and I'm sure that is just what you'd rather be doing.
A unit w/ digital in/out - esp. the ADAT format (which adds 8 convertors) is a good idea since you will likely at some point try to record a band w/ drums, which takes 8 or more inputs. Berhringer actually makes a respected unit for a couple of hundred bucks - a good bang for buck way to add to your system later once the ole bank roll is replentished.
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