Well, I have an a450n and I'm slowly upgrading it too. First off, the best card you can get get for the a450n is the Geforce 7800gs agp. This card though requires a 350 watt PSU (power supply unit). New PSU's are not really expensive (35$ range), and you should look for for one that has a 450+wattage level (or even 500+). Browse new-egg or tiger-direct or even eBay (or maybe your local computer store). Keep in mind this a450n has a motherboard that only supports an agp slot and 3 pci slots. You are better off upgrading the motherboard to a PCI-e supportive motherboard, getting a new PSU (500+ wattage or even more) and then getting a 8800gs pci-e (cheaper than the 7800gs) or better. What games are you wanting to play? Doing the motherboard/PSU/PCI-e graphic card upgrade will really be worth it if you want to play some more newer games.