With a PCI Express slot you can use just about any modern graphics card. What you are limited by is your power supply.
Post the wattage and 12v amps and we will see what you can upgrade to with the existing unit. You probably are not going to be able to go much more powerful than what you have without a power supply upgrade as well though.
Also post the rest of your system specs, CPU, RAM ect.
You have been the most help so far, but I am new to this forum and somehow my problem has been marked solved, but it isn't yet.
I did post my system specs. Thanks again