Hi, you will need to find out form dell or the actual motherboard manufacturer what CPUs are compatible with your system. Once you get a list we can advised which is the best one.
Its good to know that Intel is organising an online chat session on i core processors. If you feel like gaining some knowledge about intel processors then you can participate in online chat session on 26th March 2010. To participate please visit: http://intellivechat.com/facebook.html