What is the fastest CPU I can put into my Dell Dimension 4600 (AGP) and would it be worth the money?

I have a Pentium IV 2.4 GHz CPU right now with 512 MB RAM. I'm going to upgrade the RAM to the max (4 GB) and install a new 320 GB hard drive. I'm also planning to install a new graphics card (Radeon X1550) I mainly want to boost my performance and maybe do a "little" gaming (Sims 3, Unreal Tournament, etc) Will the RAM and graphics card be enough or do I need to upgrade the CPU? Is upgrading the CPU even worth the money?