I ended up using a 6 to 8 pin PCIe power adapter.
I measured the AC power draw of the Dell T7400 using a KillAWatt power meter.
box idle, no GPU = 185W
+ 1 CPU active +48W
+ 7 more CPUs +60W
C870 +80W idle / +95W running
GTX280 +48W idle / +95W running / +120W peak
Peak power draw with the GTX280 and 1 CPU active was 353W.
The alternative was to buy one of the dedicated VGA power supplies that fit in a 5.25" drive bay. These provide just +12V with two 6-pin and two 8-pin PCIe power connectors:
Thermaltake W0157RU 450W $70
ePower EP-350CD 350W $90
FSP Group BoosterX 3 400W $90 (only has four 6-pin connectors)
Just to get back on the bandwidth topic… I have both a GTX280 and an NVS290 in the T7400’s two PCIe x16 v2.0 slots. BandwidthTest reports:
pageable h->d 2.0 2.1 GB/s
h<-d 1.7 1.7
pinned h->d 2.6 5.7
h<-d 2.5 6.0
d->d 4.1 115.7