this is my first time seeing HP use 475W 80+ psu on its product (i never had branded pc as it was overpriced and was never intended for games)
According to this site
https://www.cnet.com/products/hp-workstation-xw4600-core-2-duo-e8500-3-16ghz-4gb-ram-250gb-hdd-vista-business/specs/
and
http://h10010.www1.hp.com/wwpc/pscmisc/vac/us/product_pdfs/xw4600_datasheet_may09_043009.pdf
you can upgrade it to 8Gb
and yes you can use standard gpu
BUT
You need to realize the cpu is no longer adequate for supporting even $180 gpu.
If you still insist on upgrading then I would recommended you to get GTX 1050Ti as the max limit of your upgrade. It should be safe for your psu since its a 75W gpu ( recomended 300W psu) and the price is not too...