What should my next gaming pc upgrade be?

Well i was just wondering what my next upgrade for my gaming pc should be? I was thinking maybe a ATI Radeon HD6850 1GB DDR5 graphics card. What do you guys think?

The specs are:

CPU:
AMD Althon X4 640 3.0Ghz Quad Core Processor

Motherboard:

Gigabyte GA-MA78LMT-S2 Socket AM3 Motherboard (8x USB2.0 Port, ATI Radeon HD 3000 onboard graphic card (DX10), 4 SATA2 Connector, 1 Gigabit LAN Port)

Ram:
4GB DDR3-1333Mhz Ram (Dual Channel)

Hard Drive Disk:
500GB Western Digital 7200rpm SATA-2 Hard Drive

Graphic Card:
512MB DDR5 Sapphire Radeon HD5670 Video Card (HDMI / DVI / VGA output)

Computer Case:
Raidmax Tornado Black Gaming Tower Case (2x Front USB2.0 Port & Audio Port)

Power Supply:
500WRaidmax SigmaATX2.0 Power Supply

Optical Drive:
LG 22x SATA DVD Writer

Operating System:
Windows 7 Home Premium 64bits Pre-installed

OK thanks i should get the card in a month or so.