Introduced: 2003
Socket/CPU: 478 (400/533MHz FSB), does not support Prescott CPUs
Chipset: SiS 651/962
RAM: DDR, up to 333MHz (2 slots, max 2GB total)
Audio: Realtek ALC650
GPU: Built-in SiS GPU, but has an AGP 4X slot
Expansion: AGP 4X, PCI, IDE, Card reader
While it is stated in some places that the fastest CPU supported is the 2.8GHz Northwood (due to chipset limitations), I have verified in my own Mega 651 that it'll happily accept a 3.06GHz Northwood (which are not cheap, in case you decide to go look for one) and work just fine with Hyperthreading (both threads are picked up in XP). A BIOS update may be required for this, however.
This system does also support Windows 98; however the radio function will not work within Windows. MSI Radio will install and start on 98, but the i2c driver required for communication with the radio module will not be installed (and might be incompatible, I've not tested this yet)
Modern-ish IDE optical drives are recommended, the ones with shorter depth. This allows more space for cable management and airflow to the power supply (which desperately needs it).
Power supply is an FSP unit, and seems well built. Capacitors were fine upon checking.
Capacitors: Unfortunately not surprising, seeing as this machine was made during the cap plague era. A couple caps had gone bad on mine, with the rest being seemingly okay. A cap map of this will be done at some point.
Power supply: While mine hasn't failed, putting this here as a concern point: If you're not going to use this system for an extended period of time, it is recommended that you unplug the system as the PSU is always running for the CD player/radio functions while the system is off. While in this state, the PSU fan is not running, which may cause the PSU to cook itself. It may be fine, but there's some paranoia here (G4 MDDs had PSUs that also never turned off and would happily cook themselves to death if left to idle, for example).
Work in Progress