Upgrade Power Supply (you might need it with a high-end video card)
http://www.pcpowerandcooling.com/products/power_supplies/selector/14a.php?model=Dimension%204550.htm
4X AGP (for the 4550)
I would check device manager and see if you have any little yellow exclamation marks.
On the lower right-hand side of the task-bar, how many little icons are running?