Hello,
 
I recently purchased the Roland Quad Capture USB audio interface and it works great!  I moved away from the Creative X-fi Titanium internal card.  I noticed the difference right away in the quality.  I am using Platinum as the DAW and am wondering if anyone knows if there is an interface that I can add that has multiple inputs for guitar, mic, etc.?  What I want to accomplish is being able to record the guitar on the first track and while playing the guitar, record the vocals into the mic plugged into the second track.  I started off with a Fostex 280 in 1990 and I used to be able to do this; play guitar and sing at the same time on separate tracks.  This helped me keep in time while playing the rhythm.  My demos are simple, mostly guitar and vocals but I am hoping that if there is an interface with multiple inputs, I can expand my demos.
 
Thanks!

If you wish to build up the song track by track, your existing Quad Capture is fully capable to do that. For the Quad, you can have input 1 plugged to the guitar and input 2 plugged to the mic, and you can either do it one at a time or both, playing guitar and vocal at the same time, recording each to a separate track.
 
If this is not sufficient, how many inputs are you looking for (assuming you wanted to plug everything at once so you don't have to re-route your guitars, since you're operating as one person) ?

^^^^ agreed. From what you have described, you may already have what you need. Take a look at input routings to SONAR.

As said , the Quad Capture has the 2 inputs so to make it work you insert 2 audio tracks into Sonar and assign the inputs.
One to Quad Capture ASIO Mike 1 Left
And one to the same but right.
ASIO names the inputs in stereo pairs so it's sometime confusing. 
Odd numbers are left and for even numbers you choose right. 
 
Interface ASIO Mike 1 left = #1 
Interface ASIO Mike  1 right = #2  
Interface ASIO Mike  1 Stereo is for a stereo recording of 1&2 
Interface ASIO Mike  3 left = # 3 
Interface ASIO Mike  3 right = #4  

As above, most interfaces will work, providing they have a preamp and a DI for guitar (the "preamp" is short for preamplifier, which brings a mic signal up to line level while a DI does the same for a guitar signal).
 
You can "stack" Roland units, as above, for more inputs.  You can get an interface w/ more inputs (2/4/8/16 are the usual numbers, going back to tape track standards).  Or you can add more inputs via digital ins and outs.  I don't know if yours has a stereo  SPDIF or AES connection (or ADAT, which is 8 digital in/outs over a single cable) but I added a TASCAM uh-7000 stereo interface via Spdif to my TC Konnekt interface because it is a higher quality convertor and has really great preamps.  And it is (relatively) cheap for the quality.
 
@

The Quad Capture features 2 analogue mic/line inputs and a stereo Spdif input.