You are going to have trouble with it. If your PC has a PCI-E interface you can easily get a cheap good card.
It is more likely though that you have an AGP card. AGP cards are a bit bigger for good ones, and the are expensive because they are no longer produced.
Even if you do get a new graphics card, you will be extremely limited on what you can do with it. Your CPU is old, and considerably weak. I have an old computer at home with an AMD Athlon 64 3200+ overclocked to 2.4Ghz and that CPU is too weak to play anything newer than Fallout 3, and Fallout 3, despite coming out in 2009, has a lot of places where the game drops to unplayable frame rates if I am not careful.
If you are only wanting to play old games, look in your case and tell us if you have an AGP or PCI-E slot. Also find a link to your model of computer. If you do this, we can help you find the best card for the lowest money that you can use.
If you are hoping to play new games, you will need to do a full upgrade.
that cpu is way too slow to play modern games. Plus it probably doesnt have PCI-E which is needed for new cards. What you need is a new pc most likely. What are you trying to play and what's your budget?
I am pretty sure the Pentium 4 won't get you anywhere. Just so you know, like I said I have a setup right now with a good graphics card an an AMD Athlon 64 3200+ overclocked by 200Mhz and it barely will play Fallout 3. If you want to play games of similar age and older, go for it. But if you want games newer you really need to upgrade.