At least according to this fairly detailed eBay listing you might be limited in what GPUs you can run in an R730. It states a 300w max per card and double width, which would eliminate both the power and physical requirements of the 3090/4090. You could run say some Tesla P40s but they would be a bit slower.
Another option would to be just buy a rack mount 4U case and say a X99 motherboard (same era CPU as the R730) which would give you a bit more flexibility in running 3090/4090 cards so long as you had a 1200w or so PSU.
There are only a handful of manufacturers of DDR4 so it’s likely some rebrand of more well known memory, or just the name of the seller.
If you have any concerns about it looks like there are a handful of sellers with regular branded Micro/SK Hynix DDR4 64GB 3200 ECC for about 90-100 as well on eBay.