Dell Inspiron 620 MT Graphics Card

I have a Dell
• Intel® Core™ i5-2320
• Microsoft Windows 7 Home Premium (64-bit)
• 6GB DDR3-1333 RAM
• 1TB 7,200RPM Hard Drive
• 16x DVD±RW Drive
• 8-in-1 Media Card Reader
• Intel® HD Graphics 2000
• Dell Wireless 1502 802.11n
• Display Not Included

Dell 21.5 HDMI LCD monitor.

Just trying to find a graphics card that will let me play StarCraft 2 on Ultra. I've only been able to use the Radeon HD 6670 on High. I know it's picky, but I am used to playing on Ultra. I don't care if I have to upgrade the PSU, but all the other cards that I have tried cause the game to be choppy on Ultra settings or suggest low settings. Please help...