Usually when you have supplementary PCIe connector to a video card the limit is decided by the psu power ratings on those connectors and not so much on the motherboard PCIe slot. I read somewhere that when bios/video card detects it can draw power from PCIe connector it will draw significantly less power from the slot. that being said I see user validated 3080 Ti in T7500 with X5675 cpu. [URL] Since 3080 Ti and 3090 are both 350 w card, you can likely run 3090 too as long as your psu is powerful enough.  The issue w T7500 or X5675 is that it is LGA1366 socket on the same platform as i7 1st gen cpu such as 990X, 980X, 980.  It is known that without overclocking the cpu, a 1st gen intel i7 cpu can significantly bottleneck modern gpu starting from GTX 970.  And the flagship 3090 is three to four generations later and close to 400% faster than GTX 970.  So if the cpu is already bottlenecking 970, 3090 will suffer extremely severe bottleneck. The gpu that matches i7 1st gen well is GTX 770 or 780.
Hi, I have an old T7500 lying around and I figured I could turn it into a GPU rendering machine. I'd like to put an Asus TUF RTX 3090 in it, which is a 350W card, peak draw is reported around 375W. My concern is that the GPU slot in Dell documentation is rated at max 300W. Not sure if it's a real limitation of the slot (after all the auxiliary power is coming from the PSU through 8pin and two 6in cables) or maybe it's just a default max figure from back when Dell never envisioned GPUs with more than 300W of power draw 
Dell T7500 and RTX 3090 - anyone tested?