I actually have a pair of XFX 256MB 8800GT's
(UK web site) which I've added a pair of Accelero S1 rev2's to.
The major problem is that, contrary to what I've read elsewhere, the speed of GDDR does make a difference, which is seemingly more noticeable at lower speeds. The cards I have default to a quite pathetic effective 1400MHz each but raising this to 1500MHz cuts frame times by up to ~10s!
The minor problem is GPU squealing, although I found that a bit of blu-tack stuck over the 'no-pcie-power-cable-connected-speaker' (don't ask) has reduced that to a more bearable level.
The total power draw is roughly ~195w for an E7200 @2GHz @0.784v and 2 * 256MB 8800GT's @ 687/752 (core/mem) which is giving, currently, 9,700ppd. However, I'm still fine tuning this hence my erratic ppd.