In regards to the processors, as long as your bios is current, you can use any Core 2 Duo/Core 2 Quad that runs at 1066Mhz FSB. I have one of these machines (as our htpc) and have a C2D E6700 (2.66Ghz) in it. For quads, Q6600/Q6700 are your only options as all the other use 1333Mhz FSB. In regards to the motherboard, these machines use BTX cases (instead of ATX) so just buying any old motherboard will not do as you will quickly realize that the orientation of everything will be backwards. So, I'd either go for a Core 2 Duo (which can be had pretty cheaply nowadays, depending on the model) the Q6600/Q6700 (which can be found but can be pricey), or try to get your hands on one of the newer dell boards, as mentioned, so you can use the new Wolfdale core based Core 2's. I have a machine with the E8400 in it and it is super snappy. 3Ghz w/6MB L2 Cache. As far as memory goes, the dell's can be picky, and if MY memory serves me correctly, this board will revert to 667Mhz operating speed for the memory when you install 8GB. I have 4GB DDR2-800 (PNY brand) in mine and have never come close to utilizing it all. You'll also get better performance memory-wise if you use dual channel mode (by having the same capacity of memory in each channel, for example 6GB would use 1x 2GB and 1x 1GB in each channel)