One thing you'll want to check before you upgrade is your power supply, make sure it can handle the load from the upgrade. I'm not sure what kind of power supply you have for your system but it's worth checking into. You may need to change it as well.
I did find the chart comparing the power consumption of a 7900 with a radeon 1900, the 7900 wins by quite a margin.
You'll also need to check what kind of slot you have for your card as in PCI, AGP or PCIe, it's most likely the latter.