Lets see...I don't think I've ever built one completely of new parts.
The 1st system I did build was a HP Vectra VE that I got in parts after a computer class (we all did). They were originally 400 MHz Pentium II, 64 MB ram, 20 gig hard drive, CD-ROM and whatever random video card was in it (some ATI Rage 128, some Trident cards, some old nVidia cards, ya never knew).
I had built mine but it felt somewhat sluggish, so I threw a Powerleap IP3/T adapter in it with a 1 GHz Pentium III, put 512 MB ram in it (finding a 256 MB+2x128 MB ram modules is easy for PC100\133 ram) and a 24x CD burner. Felt so much faster than those 400 MHz Pentium IIs.
As for one that isn't entirely OEM with an OEM case...I think that would be a...it's either a Dual AMD Athlon MP 3200+ system in a Gateway P5-100 case (kinda killed that board after a while using the stock Gateway PSU and a +12v 4-pin CPU power adapter and no 2nd motherboard power connector) or a 2.8 GHz Pentium 4 HT desktop in a card board box
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The 1st "old" computer I built was a Shuttle HOT-433 motherboard, 66 MHz 486 DX2 with 16 MB ram, 1 gig HDD, 1.44 MB floppy and a S3 Trio 64V+ video card. Never did anything with that as I didn't have a keyboard or keyboard adapter for it. It's now been upgraded and is currently a 66 MHz 486 DX2-66 (looking at getting a DX4-100), 32 MB ram, 2.1 GB IBM SCSI HDD, 12X SCSI CD-RW drive, Tekram DC-390U SCSI controller, 1.44 MB floppy, 360K floppy, 3Com 10/100 PCI network adapter that likes to run at 10 Mb/s (even though it's using CAT6 cable to a 100 Mb/s switch) and still has the Trio 64V+, as not much is needed for text-only.