off the top of my head, those Dell GX280's take P4's and the fastest CPU made was a Intel Pentium4 3.4GHz, 1M L2, Prescott CPU, 478 Socket.
Please dont just make guesses when answering questions.
The best you can use in that system is Is a pentium 4 but the p4 3.4ghz will not work. That system can only support upto a 533mhz FSB and the best I see that will work is the P4 2.8ghz 533mhz FSB