first test 8900 onboard video port without any discrete gpu installed.

R9 270 needs two black 6 pin PCIe connectors.  pls verify you have plugged in those connectors from psu.

R9 270 has DVI, HDMI, DP.  which video port do you connect to?  Do you use the same video cable and monitor for GT 730 and R9 270?

if possible test your R9 270 in another desktop pc.

Hello redxps630, first I appreciate your expert advice, with regards to your response, my XPS 8900 has no onboard video, it came with the original dedicated GPU which is the GT 730, and I need upgrade the PSU from the original 460W to 750W, the new onw has the specific connector for the new GPU which is 2x6-PIN, I did use a HDMI, also tried the DVI connection, I do use the same monitor and HDMI cable for both connections, I have no other full size desktops that will fit the card, the cards I tried with the 8900 are both tested and worked from where I bought them, I both tried Asus Nvidia GeForce GTX 770 2GB DDR5 and AMD Radeon R9 270 2GB DDR5.  I will attempt to use another OS, Win10 Pro, it might be the MOBO Drivers.  Will let you know of the result.  Thank You.

redxps630....also, forgot to mention, that under DEVICE MANAGER > OTHER DEVICES....there is a Exclamation Mark on the following: 1. PCI Data Acquisition and Signal Processing Controller 2. PCI Memory Controller 3. SM Bus Controller...I am not sure if that is the culprit...did try to look for drivers for them, but there is none, I also have that on my other Dell Desktops, but doesn't affect my use of the computers.

redxpx630.....no luck with Win10 Pro, screen is still blank despite powering up

When you say no video from R9 270, I understand it as no POST video at all.  That issue has nothing to do with boot OS.  Therefore trying different Windows etc is not going to make a difference. In this case you would like to get pre-boot Dell logo or BIOS video out of card before moving on to boot OS.

When you say your 8900 has no onboard video, I am thinking the onboard video port may be covered by a small plastic tab.  The motherboard does have onboard hdmi and DP video port.

To test pre-boot video, try disconnect ssd/hdd from motherboard. If you have a m.2 ssd on motherboard, remove it for now.  Test pc onboard hdmi video, test the old GT 730 hdmi.  If both work, then test the new R9 270 hdmi.  

I did try removing CMOS Battery for about 5mins, and put it back...but if it is that, then should it be also a problem if I return back everything to its previous setup?

I tries the onboard HDMI and the GT 730 HDMI, both worked, R9 270 same result....I do know that this R9 270 was used by Dell Alienware, so it should be compatible with Dell components.

my mistake, it is not even covered, it is just sitting there, you are absolutely correct, I have an onboard display, identical to what your image, 1xDP and 1x HDMI, I was able to address those 3 Exclamation Marks though, so no more issue with that, but, still, with everything done, win 10 or win 11, there is actually no video at all, you do see the fan spinning so it is powered up.

I am wondering if it is the PCIe Slot or the PSU....but the thing is, on the PCIe slot, that is where the old GPU is seated, and the PSU is connected to everything else, and is working fine. 

try install both R9 270 and GT 730 in blue and black PCIe x 16 slots respectively.  test hdmi video from GT 730.  If works, go inside Windows device manager to look for display adapter.  Does windows detect any AMD card at all?  upload a screenshot of dev mgr.  If amd card is shown (disregard any error), then install the AMD R9 270 driver.  restart pc, connect hdmi to R9 and see if you get any luck.  If the card is not detected at all in dev mgr, something wrong with the card.

is it the one with the Blue Tab (2-pin) (PSWD) near the Black PCIe x 16 and move it to its left side SERVICE MODE (2-pin)?

what I did instead is use onboard hdmi while the R9 is plugged in, so I can install driver for AMD, but it seems the AMD Driver can't detect the AMD GPU, I cannot use the black PCIe x16 because it is too tight, not enough space, and I think it has a lesser power than the Blue PCIex16....is yours still running fine despite software updates?

https://www.dell.com/support/manuals/en-us/xps-8900-desktop/xps-8900-desktop/clearing-cmos-settings?guid=guid-5af60142-e522-44f7-9eeb-ad16b801398e&lang=en-us

if the amd card is not detected at all in dev mgr, try replace cmos battery with a new CR2032 and clear cmos settings via motherboard jumper.  these two steps reset bios to default setting.  if card is still not detected at all, go inside bios to look for video>multi display and change settings. https://www.dell.com/community/XPS-Desktops/Dell-XPS-Intel-Multi-Display-option-in-bios/td-p/8312499

Clear CMOS settings via Mother Board Jumper?