|If you're a gamer, you never should have gotten a P4 w/AGP. As T-R-A stated, you need a newer system unless you're willing to limit yourself to older games?|
If you plan on sticking with what you have, you'll need at least 2GB RAM, a decent gaming video card, & a power supply that can handle the load. You're probably looking at about $200 US.
What were the rest of the specs of the Sempron system?