I always do this when I acquire New Device: overthink all of the things I'm totally gonna do with it (totally)
So I got a new iMac, Mid-2007 24" with a 2.4GHz Core Duo, 5GB of RAM, and a 320GB HDD.
First order of business would of course be to crack her open and upgrade the RAM and hard drive, and maybe even the CPU if it's socketed and there's something better supported. I'd also like to replace the display if at all possible - it's a glorious 1920x1200 IPS panel that still holds up today, 16 years on, but it's showing signs of... being a 16-year old IPS display. The edges are burned and vignetting, the color temperature has swayed - unevenly - towards yellow, and the backlight is beginning to leak through, leading to uneven brightness across the panel. If it's possible to source a new or like-new OEM part that's in better shape than this for a reasonable amount of money, I'd like to do it.
Of course, cracking it open in the first place is no small task. There's plenty a guide out there and it's a hell of a lot easier than later iMacs but it's not a baby's first computer disassembly by any means. There's supplies to buy and you pretty much have to set aside a whole day for the operation - because it pretty much is like doing surgery on your computer.
Once I get all of that done, though, I'd settle on an older version of OS X to run. It can actually run up to 10.11 El Capitan from 2015, an insane amount of software support from Apple. But I don't want to run that. I want to run some skeumorphic era version, probably 10.5 Leopard or 10.6 Snow Leopard. Get appropriate versions of other Apple software of the time to go with it, like the iLife suite, maybe some cracked version of Final Cut Pro, and of course the appropriate QuickTime and iTunes.
And then I'd dump my entire media collection onto the machine (although movies and shows may require re-encoding) and get like an iPod Video or something with one of those 400GB SD card RAID controllers in it and sync all my music and stuff to it. I've been wanting to get a good iPod for a while for offline media playing, and I'd definitely prefer a video-capable model if I can swing it.
Since this iMac comes with FireWire 400 and 800 ports, syncing wouldn't be an issue at all. Hell, you could probably push the full 400GB in under an hour (maybe that's a little too generous...) Suffice to say you're not stuck syncing over USB 2.
And since it has those FireWire ports... I'd also also like to get a smaller SSD in an external FireWire enclosure to run Linux off of! Because, hey, it's a 64-bit dual-core Intel, it could totally run any modern distro I imagine. This would primarily be for using a modern web browser so I could download more software to the internal volume without having to use another machine and some intermediary flash drive or something.
All in all, yeah, I've got plans for this computer. Will I actually follow through on any of them? Well... We'll see.