|The best you can do is make it a very low end gamer, but considering the age of the system & the cost involved, it's probably not worth it.|
At the very least, you would need to max the memory at 1GB, add an AGP video card, & upgrade the power supply. And depending on whether you have a mini tower, desktop or small form factor system, upgrading the power supply might be next to impossible. Even if you can pull it off, you'd still be stuck with a sh!tty P4 CPU that only runs at 400MHz FSB.
Dell™ OptiPlex™ GX240 Specs
Time to invest in a modern system.