usb keyboard controller to the pc

Yes and no.
You have to use MIDI since that's what the keyboard sends.
Keyboard>USB cable>Computer>DAW>MIDI track>Soft synth.
The sound card in the computer is an audio interface, it's just not as sophisticated as audio interfaces designed for recording.
The stock sound card will not be able to handle huge amounts of data passing through it, especially when using a lot of soft synths and audio effects. In most cases.
The latency you'll experience with a stock sound card may be difficult to overcome, almost impossible.
All that being said, there have been a couple folks who got decent results using their stock sound cards and ASIO4All as the driver. But they eventually bought recording audio interfaces once the music became too big and complex for their RealTeks.

Not so fast...


pc laptop dell inspiron  processor intel (R)  core(TM)2 duo CPU T6600 @2.20ghz/ 2.20ghz  installed memory(RAM) 8.00gb  system type 64 bit   window 7 home  hard drive 500 gig @7400 rpm  soundcard roland duo-capture  midi interface cakewalk um-2g  midi controler  roland xp50  audio device  headphones sennheiser hd497   cakewalk sonar  home studio 7 xl  music runs with in you 

As you noted, the Duo-Capture is your sound card.
It has ASIO drivers, and when in use, it will be the only soundcard/ interface recognized by SHS7.
While it is a fairly basic unit, it should have low enough latency to overcome the issues Greg mentioned.
In this scenario, you don't need the um-2g.
Since your keyboard send MIDI out via USB, the um-2g merely duplicates what is already built in.
I would suggest using the keyboards own power supply, rather than powering the keyboard via USB, if that is an option. USB power may be marginally inadequate, and can cause odd performance issues, such as lost/stuck notes, or irregular timing.
You should be able to work with the hardware you have on hand.