OK, I am going to throw final ball here.
First, lets examine what Dell says and what users say.
Example 1. My Inspiron 570 maximum CPU upgrade per Dell is Phenom II x4 945, it is official. I happened to be the first documented user to attempt to install Phenom II x4 965 BE, and it works, now no one question this fact and many people follow my steps, despite the fact that this chip is not "supported", but it works! Second, with the same Inspiron, official RAM size is 8 GB, well, one user tried and got 16 GB working, I followed and I have 2x4GB working now, can upgrade to 16 at any moment, not officially supported, but working.
Example 2. Optiplex 755 SFF, a bit smaller than 755 DT (futured in this post) and has 275 watt power supply (755 DT has 280 watt). PCIE is rated on motherboard same 25 watt, however, I serached everywhere and guess what, there is no official Dell statement about PCIE 25 maximum rating of 25 watt, so this looks fishy to me.
So, brace for impact and keep reading, this is how miracles are born.
This is life example of Optiplex 755 SFF with Sapphire 7750 low profle card -
http://en.community.dell.com/support-forums/desktop/f/3515/t/19349759.aspx?PageIndex=4 if you don't believe me, read it.
And finally, example 3. This is from official Dell upgrade site.
The Visiontek® Radeon 7750
Manufacturer Part# : 900574
Dell Part# : A6523076
http://accessories.dell.com/sna/productdetail.aspx?c=us&l=en&s=dhs&cs=19&sku=A6523076&mfgpid=192262
Compatibility (means tested by DELL)
This product is compatible with the following systems:
***TEST***OptiPlex 9010 DT
Alienware Aurora
Alienware Aurora ALX
Dimension 4700
Dimension 4700C
Dimension 5100
Dimension 5100C
Dimension 5150
Dimension 5150C
Dimension 8400
Dimension 9100
Dimension 9150
Dimension 9200
Dimension 9200C
Dimension C521
Dimension E510
Dimension E520
Dimension E521
Dimension XPS Generation 3
Dimension XPS Generation 4
Dimension XPS Generation 5
Inspiron 530
Inspiron 530s
Inspiron 531
Inspiron 531s
Inspiron 537
Inspiron 537s
Inspiron 545
Inspiron 545s
Inspiron 546
Inspiron 560
Inspiron 560s
Inspiron 570
Inspiron 580
Inspiron 580s
Inspiron 620
Inspiron 620s
Inspiron 660
OptiPlex 3010
Optiplex 320 DT
Optiplex 320 MT
OptiPlex 330 DT
OptiPlex 330 MT
OptiPlex 360 DT
OptiPlex 380 DT
OptiPlex 380 MT
Optiplex 380 SFF
OptiPlex 390 DT
OptiPlex 580 DT
OptiPlex 7010
Optiplex 740 DT
Optiplex 740 MT
OptiPlex 745 DT
OptiPlex 745 MT
OptiPlex 745 SFF
OptiPlex 745c
Optiplex 755 Energy Smart DT
Optiplex 755 Energy Smart MT
Optiplex 755 Energy Smart SFF
OptiPlex 760 DT
Optiplex 780 DT
Optiplex 780 MT
Optiplex 780 SFF
OptiPlex 790 DT
OptiPlex 9010
OptiPlex 960
OptiPlex 980 DT
OptiPlex 990 DT
OptiPlex GX280 DT
OptiPlex GX280 MT
OptiPlex GX280 SFF
OptiPlex GX620 DT
OptiPlex GX620 MT
OptiPlex GX620 SFF
OptiPlex XE DT
Optiplex XE SFF
Precision Workstation 370 DT
Precision Workstation 380
Precision Workstation 390
Precision Workstation 470
Precision Workstation 490
Precision Workstation 670
Precision Workstation 690 (1 Kw Power Supply)
Precision Workstation 690 (750 Mw Power Supply)
Precision Workstation R5400
Precision Workstation R5500
Precision Workstation T1500
Precision Workstation T1600
Precision Workstation T1650
Precision Workstation T3400
Precision Workstation T3500
Precision Workstation T5400
Precision Workstation T5500
Precision Workstation T7400
Precision Workstation T7500
Studio XPS 435t/9000
Studio XPS 8000
Studio XPS 8100
Vostro 200
Vostro 220s
Vostro 230 MT
Vostro 230 ST
Vostro 260
Vostro 260s
Vostro 270
Vostro 270s
Vostro 400
Vostro 410
Vostro 430
Vostro 460
Vostro 470
XPS 200
XPS 210
XPS 400
XPS 410
XPS 420
XPS 430
XPS 600
XPS 630I
XPS 700
XPS 710
XPS 720
XPS 720 H2C
XPS 730X
XPS 8300
XPS 8500
So, according to Dell 7750 low profile card will work in Optiplex 755 DT.
Can anyone attempt to find any arguments here?
This is called search. I stay behind my words!
So, to OP, yes you can upgrade GPU as well, I know it is too late now, but if you want, you can still to do so! Proven, works!