Glenn: I am no sound engineer, but I have some friends that are pro at scrubbing. I really can't tell you the secretes of how they do it, as many of then will not tell me all the details. I am sure the suggestions might help but let me put in my 2 cents.
The best way to get rid of noise on a track, is not have it there in the first place!
First you say it is an acoustic guitar. But, how are you recording it? Last night I listened to Grand Funk doing "Don't Look Back" on an acoustic, recorded 40 years ago, in 1973. One of the best sounding acoustic guitars I have ever heard on a recording. No computers back then, and no noise in his track.
If you are recording the acoustic with mics, be sure to use mic pre-amps. The better the pre-amp the less noise in the mic signal, and they get very expensive. You want dead silence in the mics when there is "NO" sound, before you record. For instance spend time adjustng the pre-amp properly for the sound you desire. You can place a mic in font of a stereo speaker and let a CD play as you adjust it for EQ, Threshold, input and output signal. pre, low cut, etc. It will help, to bring up the Hi-Mid on the pre-amp. The hiss is usually having too hot a mic. Point one mic at about fret 10, and the other near the bridge base of the guitar, none at the hole of the guitar. Make sure the meters at the armed track are not floating up the scales with dead silence, this is a sign they are too hot. Get the gain right on the mics, and then carefully lower the output signal compared to the in gain at the pre and mic, to achieve a recording signal from about -6 to -15 dbs when struming the guitar. By reducing the output signal along with the gain signal achieve total dead silence in the signal when you are totally quiet. One will outweigh the others ability to do this.
So now that you have that adjusted, you should see a -6 to -15 db at the armed track as you strum the guitar, and totally nothing on the meter when your are silent. Place the mics in position to how close they are to the guitar for tweaking this signal according to what you are playing. Of course noise at places in the track where you meant to be silent like tapping your foot etc...can be spliced out of the track, in clip automation or other ways. Do that last as the volume automation will prevent you from doing the mix, or you can do this and then bounce it to a new track, and remove the envelope at that point. The total dead silence factor on a mic signal beats trying to remove noise from a already recorded track. the better the original track was recorded the better the end product. You probably need to be a sound engineer to actually know all the pros amd cons of master scrubbing. Ambient sounds is another story. I assume they do both in line and mic recording and then have an engineer scrub it properly. Some of this gear gets way into the bucks.
If you are recording "line in", get a Boss noise reduction for the guitar feed, and run the out, assuming it is XLR through the pre-amp and do the same process. Also, use a Baggs or Zoom or some processor to reduce noise reduction, Fishman acoustic amps have a Anti Feedback built in that also helps. You should hear absoultly nothing in a signal when it is dead silent. Live recordings another story. Heavy Metal again, another story.
So go listen to the signal at dead silence and hear if you are achieving that to start with! Then take the above advise and you are starting to get a professional scrubbed track.
Best of luck! Free
PS: Go ahead and plop another 2 gigs of ram into that pc.