You're not running any current high end games with a Pentium 4, it's just too slow and is severely lacking in core count by today's standards, it's a single core CPU. If it's a really old Pentium 4, you might have a hard time getting a graphics card for it, as the earlier Pentium 4s use the deprecated AGP interface for graphics cards, and no card has been made for that interface since 2008.
Post your full system specs, though I suspect that you're looking at a full system replacement to run new high end games. The best case scenario for you in terms of upgrades is you have an LGA 775 Pentium 4 on a motherboard that can take a reasonably fast Core 2 Quad CPU. Even if you do have that, you're probably best off buying a whole new CPU/motherboard/RAM combo, as even the Core 2 Quads are starting to struggle a bit on newer titles.