I would use the $140 towards a new Video Card, and faster RAM.
You can get a GeForce 220 GT 1GB video card w/SLI support, for around $75.
Use the remaning $65 for faster RAM.
Since your CPU has 1066 FSB, I would step my RAM up to DDR2 1066.
(Your MoBo supports 1333 FSB, but your CPU is only 1066)
You might not get 3GB for $65, but you can get 2GB for around $50-55.
I can't see why you would want to upgrade your CPU. It might be socket 775, but still a decent CPU. Same with the MoBo.
I guess you could always upgrade to Windows 7 and skip the hardware upgrade for now.