+1 for the Focusrite interfaces.......excellent driver/customer service support (a Major factor for me). 

Uh - your Mobile Pre interface may continue to work just fine.
 
Just make sure your drivers are up to date for it, AND make sure your settings for the interface and for Sonar are appropriate.
 
Here is the link to the drivers currently available for the Mobile Pre:
 
http://avid.force.com/pkb/articles/en_US/download/MobilePre-Drivers?popup=true&NewLang=en&DocType=1450&q=MOBILE+PRE&spell=1&site=avid_en-us_default%7Cavid_en-us_pressreleases%7Cavid_en-us_default%7Cavid_en-us_pressreleases%7Cmaudio_en-us_default%7Cpinnacle_us_default%7Csibelius_us_default%7Ckbase_en-us_default
 
If you scroll down a bit, you will see the current Windows release, which includes up to Windows 8.1, so that should be just fine.
 
As far as settings, I would suggest as a starting point, for recording in Sonar:
 
Audio interface: Sample Rate of 48 k, ASIO Buffer Size of 128
Sonar: Driver Mode of ASIO, Sample Rate of 48 k, Record Bit-Depth of 24 bits.
 
Please give those a try and note that I suggest those for recording in Sonar.  Once you move on to mixing, you will likely need to bump your ASIO Buffer Size up to as high as 1024, to give your mixing plugins enough room to do what they do, for effects such as: Perfect Space and Boost 11 (and others).
 
Tracking/Recording: use low latency, small ASIO Buffer Size
Mixing/MasteringL use high latency, large ASIO Buffer Size
 
Good luck, 
 
Bob Bone
 

Hey Bob I just posted those drivers LOL :)

Well, Alex, at least we are on the same page today (don't know if that's good or bad yet, but it's a start)
 
:)
 
Did you see my PM from the other day?
 
Bob Bone
 

Thanks for reminding me, I had three PM's I forgotten about!
Ta...

I have the scarlett 6i6 and I wrote a review explaining why I chose it over the 2i2 or the 2i4. 
There is some important differences in features for, in my case only $40 more in price than the 2i4. 
 
Inputs- front panel 2 are combi for Mike/Line or instrument. ( 2i2,2i4 yes) 
Inputs -rear panel 2 line bal line  ( 2i2 no,2i4 yes) 
4 analog balanced outputs   ( 2i2 no, 2i4 yes  ) 
SPDIF in/out = your 5/ 6    ( 2i2 ,2i4 no) 
Real MIDI in/out   (2i2 no, 2i4 yes ) 
2 Headphone channels   ( 2i2 ,2i4 no) 
Mix control software, ( 2i2 , 2i4 no ) 
Mix control shines for cue mixes. The 6i6 can have 4 separate headphone mixes, 2 stereo, 2 mono. You can send any track from Sonar to these cue mixes without disrupting your Sonar mix as they will be there own buss you can add to any track. The power of the Mix Control software alone is worth $40 if you need cue mixes. 
 
I believe the pre amps will be the same on all models. They are OK for the price. Read my review for comments. 
I will say most important of all is there was a big performance jump with using the Focusrite drivers over the Tascam's ( ya, no surprise eh! ) I never get the drop outs I would get with the tascam when using certain heavy CPU plug ins now. It was the reason I got fed up one day with the tascam and got in my car and bought the 6i6 , it was pre meditated not impulse but I'm pretty stoked. And talk about solid and portable. It's made out of metal, not plastic.  
 
http://www.gearslutz.com/board/reviews/906141-focusrite-scarlett-6i6.html
 
 

Roland quad capture is another one that would meet your needs nicely.

Did somebody say Roland....?  I'm trying to remember .....Roland? Something surpressed is going on... :)

The best interface for X3.  I'd say Burl, but they are about the best "interface" for any system (if you like transformers).  On a more realistic note, the focusrite stuff makes many people happy.  The new Tascam USB unit (UH-7000) simply works and sounds superb, but is one of those new "premium" units w/ less channel count.  Quality over quanity, like the Focusrite Forté. 

V 100