I want to upgrade my dell dimension 9100 pc with a new graphics card so I dont have to buy a new computer> Have any Suggestions?

You'd be better off putting the funds toward a new or newer used system -- at almost 20 years old, this one isn't worth the upgrade (or the upgrade limitations).

It can only play old games with old Pentium D or P4 cpu.

here are some user validated graphics card in 9100

https://www.userbenchmark.com/System/Dell-Dimension-9100/1395