Are you saying you have no gps yield equipment now?

If so-we did exactly what you describe for a few yrs w/the AgLeader PF3000. If you're just getting started, this would be an OK way to go....worked for us. AL doesn't make the PF3000 any longer, but they're kind of like the JD4020 of monitors, I think. Lots of good ones around & a few for sale. Still seems to be a sellers market on them. You can usually find one on the NewAgTalk FOR SALE site:

http://talk.newagtalk.com/forums/forum-view.asp?fid=8

We hooked our PF3000 to the planter tractor w/gps & then datalogged the placement of corn/bean varieties. We then used the AgLeader basic SMS software to print maps of those varieties for post-planting scouting records. Very easy to do. Then that PF3000 would be put in the combine in the fall for harvest. Now, those electronic maps created @ planting time would NOT come up on the screen (wasn't possible), but we carried the color-printed paper copies for record. However, after you combined the crop, you could then move the harvest data from the PF3000 to the SMS software on your desktop computer & "overlay" the maps (planting & harvest) & "query" the specific areas for yield/moisture results & get a printout of that specific variety area you wanted....you control the area! It was a 2-3 step process, but still fairly basic maneuvers as far as yield mapping software is concerned, I think anyway. Worked fine.

However, spending more money will move you up to the AgLeader Insight monitor which will make colored (vs b&wh maps on PF3000 screen) maps on-screen in the tractor/combine & also display your planting maps (varieties) on the screen while combining.

But, there's nothing wrong w/getting your feet wet using the older PF3000. I think they'll be around a few more yrs yet.