As you may (or may not) be aware, I built a PC which is now surplus to requirements so I know have the option to swap this for my main PC if I want to.

I have the choice of swapping my:

Xeon X3210 (2.13 GHz - think of it as an underclocked Q6600),
Asus P5K-E.

For my:

Phenom II 920 (Overclocked to 3.4 GHz from 2.8),
Foxconn A7DA-S.

The advantages would be a speed bost (if it was faster) and also a much quieter PC and also a better case (as i'd use it as an excuse to keep the Antec 900). I'd also get a working onboard network card etc.

The disadvantage is that it's a cheaper mobo overall, and the Phenom isn't that effecient heat / power wise.

Another thing - RAID. I will be getting some new HDD's (probably 2x 7200.12 1TB) and putting them in RAID0 so if the onboard RAID is carp then thats another negative.

Intarweb reviews seem to suggest the Phenom is slightly faster, but i'm not sure.

get better HS/fans if you want silence

a thermalright tru(e) and a couple noctuas at 600rpm are your friend.

Phenom

Edit: Don't mean to be so..what's the word..heh, only posted one word, but it looks like you already know everything there is to know, my opinion is the Phenom route, but I admit I could have a slight bias.

phenom for sure

i would go for the phenom to, 1 its faster and 2 its a amd