Looks like you're using an AM2 motherboard, so we'll start there.
Grab yourself a new AM3+ motherboard, and just about any Quad core will do, Athlon II to keep costs down should be fine.
Newegg.com - AMD Athlon II X4 640 Propus 3.0GHz 4 x 512KB L2 Cache Socket AM3 95W Quad-Core Desktop Processor ADX640WFGMBOX
Newegg.com - GIGABYTE GA-970A-UD3 AM3+ AMD 970 SATA 6Gb/s USB 3.0 ATX AMD Motherboard
You'll have to upgrade your RAM as well, so grab a set of G.Skill, they're some of the best in the industry right now:
Newegg.com - G.SKILL Value Series 8GB (2 x 4GB) 240-Pin DDR3 SDRAM DDR3 1333 (PC3 10666) Desktop Memory Model F3-10666CL9D-8GBNT
Not sure if you own the OS, or if that's an OEM system, so I'll throw it in here, just in case:
Newegg.com - Microsoft Windows 7 Home Premium SP1 64-bit - Operating Systems
and you don't list your power supply, so we'll assume that you need a new one, if not, feel free to ignore:
Newegg.com - CORSAIR Enthusiast Series TX650 V2 650W ATX12V v2.31/ EPS12V v2.92 80 PLUS BRONZE Certified Active PFC High Performance Power Supply
You said you upgraded your video card, is that installed in your system right now, or have you gotten around to it yet? Let me know if the one in your post here is the upgraded one or not.
So far, your total is $431.95 before shipping, and would be a fine foundation to go from. You can subtract the OS cost ($99) if you own your own retail copy of Windows 7.