I write midi music and have a Yamaha PSR530 which uses XG.  I
particularly like the human voice, eg Yamaha "Choir" (voice #113
LSB112,prog change#52) but when played using music notation programs
(Cakewalk eg) the closest things are OOH's or Ahs's which are
terrible. I dont mind using the PSR530 for perfect p-layback, but when
I try to convert the files to mp3,The OOH's are all I can get. Please
tell me how to get the really great sounds in PSR530 without
compromising to the GM sounds for transfer to mp3 or wav files.

Try feeding your keyboard back into your line-in/mic of your sound
card. Then you will be able to record it perfectly.

Where can I obtain this midi out to line in cable?

It would be a Line Out or Speaker OUT from your keyboard (depending on
model, probably either one stereo 1/4" or two mono 1/4" or even an
1/8" stereo headphone jack (not as likely) and that to a 1/8" stereo
cable.

Radio shack has http://www.radioshack.com/product.asp?catalog%5Fname=CTLG&product%5Fid=42-2616
(1/4" Male to 1/4" Male Stereo)
You'll need http://www.radioshack.com/product.asp?catalog%5Fname=CTLG&product%5Fid=274-875
(1/4" Receptical 1/8" plug) to connect to sound card.

If instead you have Right and Left 1/4" outputs, you may want to get
http://www.radioshack.com/product.asp?catalog%5Fname=CTLG&product%5Fid=274-884
which converts 1/4" to RCA, then
http://www.radioshack.com/product.asp?catalog%5Fname=CTLG&product%5Fid=42-2551
which converts two RCAs to stereo 1/8"

But you'll be the best one to know what your keyboard has with regards to outputs. 

However, once you get your Midi Keyboard hooked up, you'll need to
figure out how to record the Line-In/Mic of your sound card as
WAV/MP3. You seem to be there already.

http://www.yamaha.com/yamahavgn/Documents/KeyboardsDMI/psr530.pdf Is
your manual. Page 13 indicates that you have AUX L and R outputs as
well as a headphone output. The second set of equipment I suggested is
probably the best for your situation.

What all this does is use your PSR530 to make the sounds, and record
them directly from the PSR530.