Have you changed the CAS latency on your ram in BIOS? This can cause the symptoms you describe. If so set them back to stock speeds.
Also, you might need a bigger power supply. Getting anything less than a 400 watt PSU is sort of a waste of time as the prices on them are about the same as 300 watt PSUs.
When looking at your mainboard youll note that an AGP slot is set about an inch more toward the center of the MoBo and is usually a different color than the PCI slots. This is so you can't accidentally plug a PCI card into it.
The MX440 is a pretty quick little card. So much so that you might want to consider upgrading to a new CPU and mainboard. A 550MHz CPU simply doesnt have the cahonies to push that graphics card. With the newer faster CPUs coming out you can pick up an AMD Duron 1.3 GHZ for about $40. Why not??? Then you can shell out $70 for a new mainboard to go with it and your new graphics card and youll have a game system that rocks
If its not broke dont fix it.