Unfortunately, Pentium D would not work here, only single core, I think the Pentium D needs the 945 chipset which is in the 520/620 systems. The 280 is almost identical to the 520/620 as far as parts go, but it has a 915 chipset. Also, You can only use Intel 5xx series CPUs.
Not all LGA775 CPUs work in GX280. For example, Pentium D and Intel Core 2 Duo are LGA775 but they will refuse to work in the GX280 (might even damage components, don't try those). This is the list of processors, validated by Dell to work in your GX280, note that for CPU 3.4 GHz and above you need a 304 watt Dell power supply:
Prescott P4 Kit, 520, 2.8Ghz, 800FSB, 1MB
Prescott P4 64bit Kit, 521, 2.8Ghz, 800FSB, 1MB
Prescott P4 Kit, 530, 3.0Ghz, 800FSB, 1MB
Prescott P4 64bit Kit, 541, 3.2Ghz, 800FSB, 1MB
Prescott P4 Kit, 540, 3.2Ghz, 800FSB, 1MB
Prescott P4 Kit, 550, 3.4Ghz, 800FSB, 1MB
Prescott P4 Kit, 550, 3.4Ghz, 800FSB, 1MB
Prescott P4 Kit, 560, 3.6Ghz, 800FSB, 1MB
Prescott P4 Kit, 570, 3.8Ghz, 800FSB, 1MB
Those are the only options according to official Dell look here for proof
http://en.community.dell.com/support-forums/desktop/f/3514/p/19266896/19458879.aspx#19458879 look for Chris post.
So as someone noted above, there is nothing serious you can do here, but to get newer PC or build your own.