You kinda already do because the P4 is a Hyper-threading CPU. I mess with some old Dell towers swapping out stuff. I have put a HT P4 in a tower that did not have a HT P4. Never tried to go to a dual core CPU. Assuming it is the original motherboard it has a PLGA775 socket. See the link below for info on your CPU.

http://ark.intel.com/Product.aspx?id=27459

Some of the older dual core CPU's use a 775 socket. But will it work. There are BIOS updates for that Dell. Some of them indicate in Fixes and Enhancements to Add support for newer CPUs. Again the Question is how new. I updated the BIOS to use the HT P4 on that tower that did not have the HT P4. I assume that the latest BIOS update contains all the previous ones too but could not confirm that one way or another. So to be on the safe side I installed each one. On the Dell logo boot screen you can see which BIOS version you have. Check out the link below to see the BIOS updates and read about each one.

http://support.dell.com/support/downloads/driverslist.aspx?c=us&l=en&s=gen&os=WW1&osl=en&catid=&impid&SystemID=PLX_GX280

Here is a couple links to the specs for your tower. Some of this might come in handy to determine if the dual core CPU you choose will work.

http://support.dell.com/support/edocs/systems/opgx280/en/ug/specs02.htm

http://www.dell.com/downloads/global/products/optix/en/spec_optix_gx280_en.pdf

Also here is a link to a dual core CPU that takes a 775 socket.

http://www.newegg.com/Product/Product.aspx?Item=N82E16819116367

But again will it work. It says the Front Side Bus Speed for this CPU is 800MHz. The specs for the Dell with a P4 say this at one of the above links: Intel® 915G Express chipset, Intel® Pentium® 4 processor with 800 MHz front side bus and Hyper-Threading and 1MB L2 cache. So it looks like the CPU at newegg I randomly chose would work but I can not really be sure. The last thing I want to do is mislead you. When I play with these Dell towers I pretty much get them for free or next to nothing. I use them to learn stuff. Most people will probably tell you to not waste your time, because this is an old outdated system, and buy or build a new one. Now if you are getting a dual core CPU for free or really cheap it might be worth doing, again if possible. That would be your decision. Intel CPU's that I have had out of towers have numbers on them. The ones I have seen have 5 characters and start with 2 letters. I have been able to research these on Wikipedia. This helps me with figuring out if they might work or not. I never really know till I try but after I research it, and I spend a lot of time to be as sure as I can, if I think it will work I try it. I am sorry that all this rambling does not answer your question. But there are a lot of variables and you did not mention a specific CPU. Once you pick a CPU then research it as much as you can and hope for the best if you decide to try. Hope some of this rambling helps. Good Luck.