Hmm... mid 2003 is pretty old-ish, so you'll probably be running an Athlon XP (assuming it's an AMD chip, if it's Intel I'd say around the 2.0-3.0 Ghz range).
Now, you have several different budgets with video cards.
You can go for a cheapie budget price (less than 100 dollars), like the card you have right now. These are the only range where you'll find PCI cards, but even those have been mostly phased out. You'll probably want to avoid these, but you CAN still find some old but still good cards. Here's one example:
Radeon 9600PRO,
Radeon 9600 XT
You then have the 100-200 dollar range. We're getting into gaming-quality cards here (the above can play games, but not the newest ones unless you turn the settings WAY down). These are mid-range cards, and they'll last you for a year or two, but you'll have to upgrade in a while. Examples:
nVidia 6600 GT,
6800GS, X800GTO
Then you have the ones that are 250 and above that will OMGWTFBBQPWN any game you throw at it. Including multiple games. At once. With max settings. On multiple monitors. While whistling dixie. These are EXCLUSIVELY PCI-Express cards (with a few notable exceptions, but these don't get the same performance

), which you won't be able to install because 2003 still had AGP back then, so I won't bother.
There's nowhere I can't reach.