Your computer is...old. I've got a similar vintage system, and they're almost on par with eachother. They're not worth upgrading much at this point, and the graphics card is going to be limited by pretty much anything else you've already got.
The interface you've got for expansion is PCI, not PCI Express. That being said, if you want to buy a graphics card for your desktop, you'll have to get something that will be compatible. Looking at Newegg,
here are your options. According to the spec sheet located below, along with the tech specs of PCI, even a very low end card will saturate your bus. The maximum transfer speed of PCI @ 33mhz (which your desktop has) is 133mb/s, whereas the HD4550 uses PCI Express v2 x16 that can handle up to 8 GB/s (64 Gb/s). This would mean that even a low end current graphics card is more powerful than anything you can put in your desktop currently.
Long story short, I wouldn't put more than $30 into that computer. Even then, it's not worth it given the performance you'd get by upgrading the entire unit. You could spend $250-$300 on a basic desktop that could blow what you've got out of the water.
Documentation + Tech Specs for your desktopFor the 4550