For your desktop? I hate to say it but this is not really the place to be asking that, being the Dell Laptop forum.
But it would depend on your motherboard, or I should say chipset capabilities, which you can find with a simple system information tool such as Aida32. But from the looks of that it is the Dimension 8300? In which case there are a few high-end P4 options in either the Prescott or Northwood (Extreme Ed if you have the cash) ranges.
But I have moved the thread to the Desktop board on this forum as it is more appropriate, where you can get some opinions and recommendations. You could also ask at the Dell Community Forum