Here's mine:
This poor E520 was going to be thrown in the trash. It was my grandmothers system. It lived most of its life inside a closed in desk cabinet running warm with Windows XP and 50 icons named random things such as "New Folder (3)". It also chugged along 2 expired antivirus and god knows how much actual malware. Every time I'd go over there I would do a quick msconfig / autoruns clean up to keep it going just a little longer. Well, it did its job. She used this system for YEARS, not once reinstalling Windows XP. Earlier this year she wanted to upgrade, so this thing was retired. I offered to take it.
Here is the poor troopers original specs:
Pentium 4 2.8ghz (with HT, so that's a plus atleast)
2x 256MB DDR2-533 (512MB)
250GB HD
Floppy Drive
56K modem (she MIGHT have actually used this before she got broadband, not sure, haha)
Integrated Graphics
Dell 305W PSU

Case badges tell the story of the struggle:

Thanks grandma for this junker. Let's see what we can do with it. I was somewhat excited I admit, since it DID have a PCIe 16x slot, PCIe 1x slot, and 4 x DDR2 DIMM slots. My initial impression was that it has potential to be decent. The manual states it runs an Intel 965 based chipset with a FSB of 800/1066. That's a GREAT start. That means we might be able to run Core2 ! It also states a maximum of 4GB in 4x1GB layout, but we all know how manuals can be with their somewhat outdated information, so I had hopes I could push it to 8GB, a theory reinforced by the wiki page showing 965 chipsets can run 8GB.
My Core2Duo proved a success and booted right up. But I decided to take it a bit further and throw in a Core2Quad because why not? SUCCESS! It runs great! The only problem was that it was running HOT. I had to upgrade the heatsink. This heatsink with it's heat pipe goodness is from a different Dell system and is labeled J9761:

Installed and ready to go:

And you know what? We don't really need a floppy drive right now, and the manual shows it has a "FlexBay" header on the board - ie a USB 10 pin header for card readers, so lets throw this guy in:

The funny part about this card reader is the USB labels are SS ie "SuperSpeed" which is reserved only for USB 3.0. This is a USB 2.0 card reader with a USB 2.0 header, so it's labeled wrong, but regardless I thought that was amusing. Looking better:

Now, the GPU was an interesting problem. Since it's a BTX style motherboard, everything is flipped upside down. This system only officially supports single slot GPU's, any dual slots won't fit or would interfere with the CPU heatsink shroud, here's a good pic showing the issue:

But lucky me, when I bought my Radeon HD 4830 years ago, I got the MSI version with the orb style cooler. I cut the mounting bracket in half to make it "single slot".

I also added a nice little 500W PSU and an Intel Pro/1000 Gigabit nic. So now here we are:
Grandma's tricked out Dell E520:
Intel Core2Quad Q6600
8GB DDR2-800
1TB HD
Radeon HD 4830
Card Reader w/ 4 USB
Intel Pro/1000 Gigabit Nic
500W PSU
Windows 7
It now has no problems doing modern tasks, with a modern OS. It can play most 2012 and under games nicely and can multitask like a beast now with the quad and 8GB ram.

Living a peaceful cool life now as a living room computer, don't mind the desk, it's temporary:
