I have an Optiplex 745 low profile Desktop
Even if I could use a full size card their is no way the Dell PSU
could handle a HD6950
the 740 is very similar
if it is the stock PSU
it is probably no more than 305w continous max which
is equal to about a 350-400w
the stock Dell doesnt have the 6 pin connector in those models
as far as I know
Without doing a PSU upgrade the most gpu i could put into my
745 was a HD 5670
if you changed PSUs you might be able to do it
BUT your CPU (amd athlon64 x 2 probably) would be a huge "bottleneck"
for that HD 6950
really couldnt take advantage of it
The HD 5770 with a PSU upgrade would be about the best that wouild work
but I wouldnt do it
really for the cost of a HD 6950 you could do a new system