Hi! I'm looking for a new GPU that has RTX (for gaming and 3D freelance jobs.

I have a ROG STRIX B450-F GAMING, Ryzen 7 2700, and a Sapphire Radeon Nitro+ RX 580, and 850 Watts of power.

Will a NVIDIA 4090 RTX be too much for my setup? I think my MBoard supports only PCIe 3.0, but I've heard that it may support a 4090 with no problem

I built a new system back in May 2019, and have consistently had issues with the MSI Radeon RX 570 Armor 8G OC GPU. Anyone who has/had this GPU and had any issues will know the multitude of problems I have had... Screen flickering, black/green/blue/you name the color screens, freezing up, disconnecting from display, rebooting itself, etc....Anyway.... After eight months of trying to resolve the issues I'm done and sending the GPU back to be hopefully incinerated and die the painful ugly death it deserves...So I have two GPUs here I can use while I am waiting for MSI to refund my money, but I'm not sure if either of them are compatible with my B450-F motherboard. I looked all over the Asus website but could find nothing about it, and searches in other forums yielded no concrete information. What is confusing me is this info, which is the only info I have found about it:AMD Ryzen™ 2nd Generation/ Ryzen™ 1st Generation Processors2 x PCIe 3.0/2.0 x16 (x16 or x8/x4)AMD Ryzen™ with Radeon™ Vega Graphics Processor1 x PCIe 3.0/2.0 x16 (x8 mode)AMD B450 chipset1 x PCIe 2.0 x16 (max at x4 mode) *13 x PCIe 2.0 x1. The AMD Ryzen designations are what are making me wonder if one of the two GPUs I have are compatible.

The two GPUs I have are:

Saphire HD 6450 1GB PCIe 2.0 x16 (64 bit memory bus, DDR3 memory) which supports audio passthrough via HDMI.

and

BFG GeForce GTX 260 MaxCore 55 PCI Express 2.0 x16 (896MB GDDR3 SDRAM, 448 bit bus width ) which I will need to connect to my TV/Monitor via DVI to VGA adapter.

The Saphire is of course newer, and I can have audio via the HDMI. The GTX I will need to figure out how to get audio, since I don't have an audio card... But was wondering if I could use the HDMI port on the B450-F for passthrough audio (I don't have an S/PDIF cable and don't want to buy one because this is a temporary setup)... But I want to make sure if either of these are compatibly with my MB before trying them and screwing something up on the MB or elsewhere...

My current system specs (without the RX 570):

CPU: Ryzen 5 2600x

RAM: 16GB Corsair Vengeance 3000MHz

Motherboard: ASUS ROG STRIX B450-F Gaming

BIOS Version: 2901

PSU: Seasonic Focus Gold 550FM 80 Plus Gold

Display: Samsung 50" UN50NU7400 4K via HDMI

OS: Windows 10 Home 64 bit 1909 Build - 18363.535

Also, if anyone has suggestions for a new GPU (that I won't waste months trying to get to work properly) within the $260 price range, that would be greatly appreciated as well.