If this is the PSU http://support.gateway.com/s/Servers/9220Server/WMEHP-E4008FWR/WMEHP-E4008FWRsp2.shtml
they should all work. I will like to add this HD5770 card to the list though, because it is better than the others http://www.newegg.com/Product/Product.aspx?Item=N82E16814161338
In performance order then from best down HD5770, GTS450, HD5750, HD5670 and GT240 If there are space constraints then the lower the power the more compact the card is.
For ease of installation then another Nvidia card will run on the same drivers as the 8500GT although a driver update would be needed.