I use a mirror-door dual 1GHZ with the upgraded power-supply, and it works great - my only warning would be IF you buy a mirror-door, make sure it has the newer power supply - the older one was referred to as "The Wind Tunnel" because of the noise it made.
More speed is gonna be better w/ soft synths and stuff, but in terms of sheer bang for the buck to replace your 400 I would get a 733 Digital Audio. Not the Quicksilver one but the one that looks like your 400. Probably under $400 these days on ebay.
I have MDD Dual 867, QS 933 and Grey 466. The reason I would recommend the 933 is because it's the fastest stable machine that runs relatively quiet. My MDD makes a huge noise. To upgrade the PSU is expensive, so I use it for Cubase and some virtual instruments with a slower fan. The problem with the MDD and Mix system is that they end up running very hot. I had all sorts of problems with 4 mix cards and the PSU in the Dual 867.