According to the Dell website, the fastest CPU you can use in your system is the AMD Athlon X2 6000+. It has to be the 89 watt part, and not the older 125 watt part. If you can run the CPU, you can run the 6400+ which is a bit faster and will run your 800MHz memory better. Again, it has to be the 89 watt part, unless you upgrade your power supply. If you do get an add-in graphics card, then for sure make sure you upgrade your power supply. You will also need to update the system BIOS with their latest BIOS for your motherbaord, before you replace the CPU.
If you need tips on replacing the CPU and setting up Windows after, click the link ontop of my signature for the mult-core upgrade guide. Hope this helps you out!
Multi-Core Upgrade Guide
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