Buy a new case and power supply, as a compaq has an unreliable power supply already but if it is only a p3 the watts won't be to support a 6600gt or athlon 64.
Is your OS available on CD/DVD, because if it is pre-installed on the Hard Drive it won't work on the DFI. You'll need new ram too, to boot....The more important question is what will you won't need. Also the Compaq Cases are impossible to mod, trust me i just had to buy a whole new rig because my compaq rig from 2003 was already dated...
The Power Supplies aren't just like brand name supplies like ANTEC or ZALMANN, they are custom fitted and also the connectors to the motherboard aren't standard as well...
There are only Two things in life i regret doing... 1. Selling my Soul for Cash.... 2. Buying a Compaq.... I should have kept the Cash, huh.