Pondering :
http://www.dell.com/downloads/global/products/optix/en/opti_755_techspecs.pdf
Page 6 shows the power supply size and page 28 shows the Desktop Tower (that lays flat) is the one with the 108w which is the combined Max Combined +3.3v / +5.0v Power.
Upgrading the power supply isn't looking good.
Thanks for the link. Page 28 in that link shows the desktop has a 280W PSU. I think the OP was confusing the 108W with total wattage. Nevertheless, a single 16A +12V rail won't be enough for a decent upgrade.