I may catch some flak for posting these because they're probably considered too new to be "retro" by many, but they represent significant milestones in my PC journey throughout early adulthood.
First, an ASUS P5N32-E SLI socket T (775) motherboard, which is the same board I had when I built my first Core2Duo PC with an Intel E6600. I still have that same E6600 CPU but I might stick the QX6700 CPU I currently have in another PC in this one, not sure yet. Also not pictured are two brand new Nvidia 7950 GTs which I will run SLI. This will be a mostly period correct 2006-2007 system running XP.
Next, an ASUS Rampage II Extreme socket B (1366) with an Intel i7 980X and 12 GB of Corsair Dominator DDR3 1600 MHz triple channel RAM and the Creative audio PCI-E interface card. My original 1366 PC I built around 2009 was an i7 920 in probably a midrange ASUS board, but I wanted something more high end this time around since parts are relatively cheap. I don't have a video card chosen for this one yet. This will likely dual boot XP/7.
Next, an ASUS P8Z68-V Pro socket H2 (1155) with an Intel i5 2500k and 8 GB DDR3 1600 MHz dual channel RAM. This motherboard and CPU are the same ones I had when I built my 2011 PC. I bought three matching Nvidia GTX 580s for this build but I may only use two in SLI and shelf the other one. This will dual boot XP/7.
Finally, some additional Corsair Dominator DDR3 1600 MHz triple channel RAM, 12 GB total, purchased separately.