Well, after much struggling with Steam and disk space resulting in the complete loss of all my Steam apps and the repartitioning of a hard disk to support the space requirements, I finally got GTAV PC installed about 24 hours late. Not a GTA issue, so I'm not holding it against the game.
My PC specifications are shown here. It was built in 2010 and at the time was a medium-high spec machine built to a "bang-for-the-buck" budget. Nothing was top of the line hardware. A new medium range (<US$200) video card was added last week in anticipation of the requirements of GTAV, replacing the original 1GB HD5770. I do still plan to benchmark that card to see what it can do. Apart from that, this is very much a 5 year old system.
On first run, the game was pre-configured with medium-high settings in all of the graphics options and the prologue played through at a pretty consistent 50-70 fps.
So I naturally turned everything up as much as I could to see whether it would be playable. Apart from MSAA (which works but due to a bug in the video driver on all R9 280 and 280x cards that leaves an annoying small rectangle of artifacts in the lower right corner so I have turned it off) all the standard options are as high as they can go.
Some of the advanced graphics options are also turned on.
My display only supports 1080p, no 4k display in my budget this year, nor is a PCIe 3.0 motherboard (so the new card is bottle-necking at PCI-e 2.0).
I have now played through to Nervous Ron at these settings. Frame rate has dropped under 20 a couple of times but generally sits at 25-30 fps. Completely playable. Video card never gets above 65 degrees C and fan never over 50%.
So far so good.
TLDR: A 5 year old PC with a 2 year old GPU can run GTAV at PS4/XBox1 quality and 30fps. Or it can run at PS3/Xbox360 quality at 60fps. But it can't do both.
I'll add some observational comments in the next day or so about the observed differences between PS3/PS4/PC versions which I will try to have running side-by-side-by-side this weekend.