Here's what I use: Out of RP50 I use a stereo 1/4 to 1/8 adaptor plug. I then use a stereo 1/8 Y-splitter. Out of one side of the y I connect headphones to monitor output sound real time so that there's no playback/latency issues. The other end of the y I run a double male 1/8 stereo line to the aux audio input of my PC.

This setup works well for me. The only problem is that I have to keep one side of the headphones tilted off my ear so that I can keep along with already recorded tracks as I lay down a new one. But it's not much of a problem.

Good luck! Keep in mind, tho, that not all 1/4 to 1/8 adaptors are the same. The first one I had came with the Sony headphones I bought, and works perfectly. I lost that one for a day, and bought a replacement at Radio Shack that didn't work right (only one side audio), and then found my original plug the next day.
^ yea that shoudl work but eventually you may want to upgrade to an interface like a Toneport or something similar.

Cheaper - you could buy a small Behringer mixer to feed the two outputs into and monitor it all with one set of headphones. I do this but with my PA setup which is a mini mixer built in.
Here's exactly the way my band recorded:

1. Bought a Behringer UB1202 mixer used for about $50.
2. Bought a 1/8-1/8 cable and added the 1/8-1/4 jack on one of the ends.
3. Plugged the 1/4 into the Control Room Out jack.
4. The 1/8 straight into the line in jack in the sound card.

You can then connect an electric guitar using an amp's line out jack to the mixer, allowing you to record on the computer.

I then "bought" Adobe Audition 2.0 and recorded like that. I got my band's demo this way and it wasn't half bad.

http://www.myspace.com/fypband for clips.

Equipment Used:
Behringer UB1202: $50
Cable+Jack: $8
Studio Projects B1 Mic: $120
Shure SM58: $10 (Got lucky)

Mic'ed a drum set by placing the sm58 on the bass drum and the studio projects about an arm and half's length from the snare, directly between the high hat and the middle of the drumset. The bass guitar was plugged straight into the mixer.

The guitar was plugged into a Traynor YCV40 and mic'ed by the SM58, and then for some parts, we used a Vox Valvetronix AD30VT from the line out into the mixer.

Good luck!