his computer has a 1 PCI Express x16 (Gen 2.0)
u can put almost any card
i have the same slot i put up 7870 XD its a great card can run everything at max !
u can use any card depends on ur budget
but if u have a higher budget go for 7970 or 7950
hope this helps
go for any of these threee if u go lower u would still be able to run games but i don't know if could on ultra