You have a fairly old system that is limited to AM2+ CPUs. It has a PCIe 1.1 slot but you can use video cards with PCIe 2 or 3 because PCIe is backwards compatible. You aren't limited on Graphics memory.
The motherboard chipset may cause compatibility problems with AMD cards higher than HD4000 series. You should see if anyone else had success with your type of system. I would use a conservative Nvida card. Your power supply is only 250 watts so that would not power most video cards. It should be replaced.
EVGA 02G-P4-2645-KR GeForce GT 640 2GB 128-Bit DDR3 PCI Express 3.0 x16 HDCP Ready Video Card
http://www.newegg.com/Product/Product.aspx?Item=N82E168...
System Requirements:
Minimum of a 350 Watt power supply. (Minimum recommended power supply with +12 Volt current rating of 20 Amps.)
Antec VP-450 450W ATX 12V v2.3 Power Supply
http://www.newegg.com/Product/Product.aspx?Item=N82E168...