Suggest Good Graphics Card for this System

Hello,

I have a ZT Systems Affinity with the following:

Windows 7 64
AMD Phenom™ II X4 Processor 945 (3.0GHz)
AMD 780V / SB700 chipset
8GB DDR2 SDRAM memory (4 x 2GB, shared)Integrated ATI Radeon™ HD 3100 graphics with DVI

I would like to upgrade the graphics card. The only game environment I use is Second Life. I understand from reading these threads that I am limited because of a 300W power supply? Are there some graphics improvements I could make with a simple plug in of new graphics card?

Thank you.