According to Intel, all/most of these should work in 915G-based boards, some of the later ones may require a BIOS upgrade to work so YMMV, depending on Dell:
- Processor Family Processor Number Processor Speed Bus Frequency L2 Cache Size
- Intel® Pentium® 4 660 3.60 GHz 800 MHz 2 MB
- Intel® Pentium® 4 650 3.40 GHz 800 MHz 2 MB
- Intel® Pentium® 4 640 3.20 GHz 800 MHz 2 MB
- Intel® Pentium® 4 630 3 GHz 800 MHz 2 MB
- Intel® Pentium® 4 570J 3.80 GHz 800 MHz 1 MB
- Intel® Pentium® 4 560J 3.60 GHz 800 MHz 1 MB
- Intel® Pentium® 4 560 3.60 GHz 800 MHz 1 MB
- Intel® Pentium® 4 551 3.40 GHz 800 MHz 1 MB
- Intel® Pentium® 4 550J 3.40 GHz 800 MHz 1 MB
- Intel® Pentium® 4 550 3.40 GHz 800 MHz 1 MB
- Intel® Pentium® 4 541 3.20 GHz 800 MHz 1 MB
- Intel® Pentium® 4 540J 3.20 GHz 800 MHz 1 MB
- Intel® Pentium® 4 540 3.20 GHz 800 MHz 1 MB
- Intel® Pentium® 4 531 3 GHz 800 MHz 1 MB
- Intel® Pentium® 4 530J 3 GHz 800 MHz 1 MB
- Intel® Pentium® 4 530 3 GHz 800 MHz 1 MB
- Intel® Pentium® 4 524 3.06 GHz 533 MHz 1 MB
- Intel® Pentium® 4 521 2.80 GHz 800 MHz 1 MB
- Intel® Pentium® 4 520J 2.80 GHz 800 MHz 1 MB
- Intel® Pentium® 4 520 2.80 GHz 800 MHz 1 MB
- Intel® Pentium® 4 511 2.80 GHz 533 MHz 1 MB
- Intel® Pentium® 4 506 2.66 GHz 533 MHz 1 MB
- Intel® Pentium® 4 505J 2.66 GHz 533 MHz 1 MB
- Intel® Pentium® 4 505 2.66 GHz 533 MHz 1 MB
- Intel® Celeron® D 346 3.06 GHz 533 MHz 256 KB
- Intel® Celeron® D 345J 3.06 GHz 533 MHz 256 KB
- Intel® Celeron® D 340J 2.93 GHz 533 MHz 256 KB
- Intel® Celeron® D 336 2.80 GHz 533 MHz 256 KB
- Intel® Celeron® D 335J 2.80 GHz 533 MHz 256 KB
- Intel® Celeron® D 330J 2.66 GHz 533 MHz 256 KB
- Intel® Celeron® D 326 2.53 GHz 533 MHz 256 KB
- Intel® Celeron® D 325J 2.53 GHz 533 MHz 256 KB
- Intel® Celeron® D 320 2.40 GHz 533 MHz 256 KB
- Intel® Celeron® D 315 2.26 GHz 533 MHz 256 KB
- Intel® Celeron® D 310 2.13 GHz 533 MHz 256 KB
If you are seeing 50-100% CPU usage when you aren't using it heavily, then you have something running out-of-control in the background.
Check for malware, and perhaps update system drivers.
You can figure out what's busy by using utilities like Windows' Performance Monitor, or MS' Process Monitor.
If you've had the system a long time and you can't identify the source of the mystery CPU usage, it may be time to consider a wipe and reinstall of Windows.