[Wrench97]

CPU speed is the biggest issue, overclocked to 3.0-3.2 the Q6600 can still run pretty well but that unfortunately isn't a possibility on a oem Dell board, upgrading is going to take a Case(the 420 uses a BTX not ATX board) motherboard(The Intel 775 socket is end of life with only the G41 chipset left on the market) CPU and Ram(all new boards are DDR3).

 

[Wrench97]

Yes the Dell case will only take the now obsolete proprietary Dell BTX motherboards.
The shape of the board and the front panel header connections are different then a Retail ATX board.