Yeah... you'll never get good sound with a single mic.
The simplest route (not the best, but it will be much better) is to use two mics, one on the guitar, one for voice... same rig you'd set up for a live gig. You can do this for any camcorder that takes external mics, though true condensors are an issue if you don't have a high-end camcorder with XLR inputs and, thus, phantom power available.
Another improvement can be (depending on the sound you're after) a pickup on the acoustic, rather than a mic. I do mean a real acoustic pickup... some of the pickups for acoustic use mics of some form, and give you that true acoustic sound... others use electric-style magnetic pickups and, naturally, don't carry along as much of the acoustic sound. The advantage here is that you get little or not outside sound on the guitar track. So you can sing all you like, and the guitar track stays pure.
For guitar, some people like dynamics, some condensers, and some ribbon or piezo mics... each have their signature sound. For voice, I definitely like the large diaphram condensor above all others, at least those I've had a change to use. I own about a dozen mics, two decent condensors.
If you go to PC recording, of course, you get to my next suggestion, which is studio-style recording. For this, you lay down the instrumental track, then layer your vocals over it. That's pretty easy to do on most PCs today, even with free software like Audacity. This gives you the ultimate control over your sound, lets you play multiple parts. I do this.. guitar, harmonica, and a couple of vocal tracks, and it sounds much better than anything I could get "live". Curiously, though I love the sound of my Martin D-15 live, the best acoustic recordings I get are from my Variax, pretending to be a Martin D-28 (I'm recording through a Floor Pod and USB, the Variax actually has piezo pickups, so the sound you get is based on whatever DSP program you're running). But I digress...
The basic idea of a "USB" mic is that you have a conventional mic which also incorporates what's essentially a USB sound card. The links you posted didn't work for me (even the added ones), but I'm guessing you're looking at the AT2020 series. The "USB" model is simply adding a 48V bias circuit and USB A/D converter to the base model AT2020. Anyway, that looks like a decent mic for the money.. way less than any of my condensors, but it's also true that the technology has been improving lately, and prices falling. I have not hear anything bad about Audio Technica.
Personally, I would NOT get the USB model. You might save a little money.. if you're intent on computer recording, you have to get the sound in somehow, and sure, you'll pay a bit more, maybe, for a recording interface with XLRs and phantom power (my Tascam US122 interface ran about $150, years ago). But the stand-alone mic is far more flexible... you could use it on stage, hook it to a mixer or a higher end camcorder, etc.
And I've seen a few of these little "dongles" that might be kind of cheap.. basically, a little thing with XLR on one end and USB on the other, which "convert" an XLR mic to function as a USB mic. Here are a couple:
http://pro-audio.musiciansfriend.com/product/MXL-Mic-Mate-XLR-to-USB-Mic-Interface-WPhantom-Power?sku=270707
http://www.engadget.com/2008/11/07/blue-microphone-presents-the-icicle-xlr-to-usb-audio-interface/
This might actually save you money, and you get more features.. the "MicMate" offers 44.1kHz and 48kHz audio, while the USB AT2020 is only 44.1kHz.