About circle of fifths...

This question could lead you in all sorts of directions.  I'll take a stab at getting things started.

When you speak of the circle of 5ths (4ths) you're really speaking of a tool that identifies the relationship between Key Signatures.  At the top of the circle is the key of "C".  The notes available to you within that key are C,D,E,F,G,A,B,C - i.e., no sharps, no flats.  To move around the circle, select the 5th scale degree - in this case "G", use G as your root and add a half step to the 7th scale degree, in this case, F becomes F#.  The notes available to you in the key of G are therefore G,A,B,C,D,E,F#,G.  And so on around the circle.

When your working out chords available to you in a given key, you'd go thru a process of "harmonizing" the major (or minor) scale.  Without getting into the process of stacking thirds to build chords within the scale, just know that the chords of the harmonized scale follow the pattern, Major, minor, minor, Major, Major, minor, diminished, Major.  So, the Chords available to you in the Key of C would be CMajor, Dminor, Eminor, FMajor, GMajor, Aminor, Bdimished,CMajor.  Extended chords (like 7ths, 9ths, etc) follow much the same pattern except the V chord, instead of being Major becomes Dominant.

Pretty complex stuff if your new to it.  I'd suggest you do some reading on scale and chord threory.

If you're moving down by fifths (i.e. B-E-A-D-G-C-F etc) you can use dominant 7 chords for all of them. I don't exactly know why it works tonally but it works for any chord moving down a V.

For instance, you could change the common I-vi-ii-V progression (C-Am-Dm-G) into C-A7-D7-G7. You could even make it C-A7-Dm-G.

Carrying on from what burgermeister said, we can simply use the notes from the given key. Let's look at part of the progression for "Hello" by Lionel Richie. Yeah, I'm a tool for making you listen to him but it's a good example.

This part starts off with the line "I can see it in your eyes".
Dm7 - G7 - Cmaj7 - Fmaj7
Bm7-5 - E - Am - A7

The song's in Am, which is one reason why we have the E major chord (V) in the bottom line. The other is because A is a fifth down from E. This allows us to use the major version of the chord. The same principle applies to the A7 in the last measure. It leads back to the Dm, which is a fifth down from the A. When we change any chord (apart from the V) to a dom7, we call it a secondary dominant if it leads to the chord a fifth below.

This example is also good because the chords are built upon notes from the key of C. After all, Am is the relative minor of C.

Moving up by fifths (i.e. C-G-D-A-E etc...) is a different story. Right about now I'd normally be asking Noteboat to help me out, but...

Hey Noteboat, help me out.

Moving up by fifths is the same as moving down by fourths... if you move around the circle of fifths in a clockwise direction, it's a circle of fourths.

It's kind of a one way street, though, at least as far as extended progressions go.  When you play a 7th chord, the sound is unstable... incomplete... it has a tension that wants to resolve somewhere.  In the key of C, you have:

G-B-D-F.... to C-E-G

To simplify this just a tad:

The B is the leading tone of the C scale, which tends to want to move 1/2 step to the scale root.  You've also got G; most chord progressions (other than chromatic movements) have at least one common tone between the two chords, so this one stays G... finally, you get rid of the D and F, and add E, and you're at a C major.

On the other hand, if you go

G-B-D-F.... to F-A-C

You've still got the leading tone moving up to C, but you're treating F as the common tone.  It's certainly a chord change that works -- it's in darn near every blues progression -- but it doesn't give you the sense of finality that V-I does.  In a 12 bar blues progression, the IV then moves to I to complete what your ears are anticipating: ending up on the tonic.

When you use secondary dominants, the V resolves to I, but you change I into I7.  This gives you the resolution to the triad, with one extra note tacked on:

G-B-D-F.... to C-E-G-Bb

The new chord wants to resolve further, because of the tension from the b7; moving down another fifth puts you in the key of F.  You can keep going as long as you like, although moving through more than 2 or 3 keys at a time will get sort of boring :)

The reason the V chord can be a dominant 7th in a minor key has to do with altered minor scales.  Here's the two-octave A minor harmonic scale, with every third starting at the fifth in bold:

A-B-C-D-E-F-G#-A-B--C-D-E-F-G#-A

The bold notes add up to E-G#-B-D, or E7.

thnx peepz, it helped. I'll keep on asking whatever comes in my mind.  :)