The Intel Pentium D 945 is the maximum you 'practically' can use, simply because of the thermal 'and acoustical' impact and that requires already that you have the copper-variant of the heat sink (at least if you want to drive it a work- or game- -day long).
The Pentium D series are true Dual-Cores, but are effectively slower than the Intel Pentium 4 HT series, because the scheduling used in Windows 7+ is ineffective|too slow.
Under Windows 7+, both deliver nearly same results. Under Linux, the Pentium 4 HT with 3.4 GHz delivers about 15% better performance and produces far less heat and can be run with standard aluminium heatsink.
It is possible to run a pin-modded Core2Duo, but it wasn't possible to mod the BIOS, so that it's different featureset (SIMD, etc.) all can be made usable.
The most efficient Windows OS on the Gx620 is Windows 8.1 Industry Pro, next best is Windows 10 Enterprise LTSB 2016, Windows 7 is an abomination if not trimmed down to the bare minimum and in every detail configurably possible; at least if you eventually decide to start an application.
For Office use or Entertainment, they are still more than potent enough, a Nvidia 730 is the lower bound and enables 2k without a problem, a 750 makes 4k handsome. Again, in combination with the Pentium 4 HT 3.2 or 3.4 GHz quiet enough to sit aside the TV. A 960 is the max i would recommended, because as we all know Windows buffers the graphics cards RAM to the main RAM.
The RAM is the moot mood point of the GX 620, simply because the chipset that was implemented on the motherboard does not support addressing more than 4 GiB RAM.
Another use case it does well is HomeRecording and Arrangement. The Soundblaster Live|Platinum|FX Series are all working perfectly fine with the inerts of the GX 620, as if it was built around them.
Hackintosh is possible, too with up to 10.7 or .8 i think, the highest is a modded Yosemite, which is a torture, because the machine does not meet the SSE requirements, so they had to be dummied. All in all, regarding Hackintosh: Not advisable.
It is a pretty decent machine, when it comes to make use of it for oldschool Windows 98 SE SP 3, the machine hits - so to say - the critical mass. So far the maximum performance for Windows 98 SE that can be pulled, without special-pill-in-the-rabbit-in-the-hat-tricks, possible because asides the 4 SATA there i also an IDE-bus present, allowing two drives to be attached.
Last thing to mention, because it serves well, is DOS and OS/2 tooling, especially Instrumentation and Control, just because all oldschool standard connectors are existent and with a microdrill it is possible to cut the air-grid to place in the optional PS/2 Keyboard and Mouse port + 2nd serial RS232 combining slot-blind.
Hmm, what else .. Ah, UEFI via EDK II runs fantastically, so it is a nice complement if all the machines in-House are all UEFI-only, but one wants to test against generic UEFI and BIOS.
Yeah, well .. that's what actually came to my mind.
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Hope it helps. Re-use 'dat! Thanks for reading.
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