Mr. goducks,
I will repeat what I said earlier, it's strongly advised not to have your gaming machine and business machine one and the same.
Your old HP Vectra should still be working fine as a standard business machine. It has plenty of horsepower for Office. It does not have enough horsepower for gaming, even with a video card upgrade.
In your new machine, you need to open the case and confirm you have an AGP video card slot available, or PCI Express, you may not, some business oriented machines have onboard video and that's it.
If it does have an AGP slot (the short, usually brown, one at the top of all the PCI slots) or PCI Express, (see your system documentaion), and you insist on having it as a gaming AND business machine....
An ATI Radeon 9600 XT, 9800, 9800 Pro should work fine. Several Nvidia based cards will work well too.
Go to Newegg Video cards:
Newegg Video Cards and put your minmum and maximum price in the search box on the left. I suggest a minimum price of $150 to get a good card for your new AMD system. Be sure to use either AGP or PCI Express in the filter boxes above the price boxes depending on what type you have.