8GB RAM won't be an issue. I have 8GB RAM in a machine (Retro 4 in my signature) that dual boots XP Pro SP3 and Vista Business SP2 (64-bit). The GPU is a GTX 580 3G, no issues in XP, it is just there and works flawlessly.

Why not use a boot manager? I found them always quite comfortable. Currently, I'm using Boot-US, which is free for personal use with minor restrictions (although I have the full version). But there are a lot of alternatives.

Regarding the GeForce 6. What do you mean with "ShadowBuffers"? Are you referring to the original Splinter Cell? Because then only the GeForce 3 & 4 will give you the original PC experience, even the FX has occasional issues with the shadows, and all GeForce series after the FX are incompatible and you'll end up having to use the fallback mode even on your GeForce 6...

voodoo5_6k wrote:

8GB RAM won't be an issue. I have 8GB RAM in a machine (Retro 4 in my signature) that dual boots XP Pro SP3 and Vista Business SP2 (64-bit). The GPU is a GTX 580 3G, no issues in XP, it is just there and works flawlessly.

Why not use a boot manager? I found them always quite comfortable. Currently, I'm using Boot-US, which is free for personal use with minor restrictions (although I have the full version). But there are a lot of alternatives.

Regarding the GeForce 6. What do you mean with "ShadowBuffers"? Are you referring to the original Splinter Cell? Because then only the GeForce 3 & 4 will give you the original PC experience, even the FX has occasional issues with the shadows, and all GeForce series after the FX are incompatible and you'll end up having to use the fallback mode even on your GeForce 6...

I thought PhilsComputerLab reported Shadow Buffers in Pandora Tomorrow as working on a 6600 with the 66.57 drivers?

TheAbandonwareGuy wrote:

I thought PhilsComputerLab reported Shadow Buffers in Pandora Tomorrow as working on a 6600 with the 66.57 drivers?

OK, I don't know about that game. I assumed you were talking about the original Splinter Cell.

The only thing I'm worried about is Windows XP refusing to boot on a machine with nearly 12GB total memory between System and Video RAM as IIRC video memory counts against the 3.25GB RAM limit on Windows XP.

This does not work exactly this way and there's no exact 3.25GB RAM limit inherent to XP (even without PAE hacks).
Usually the available memory under 32-bit Windows is determined by the starting address of the video card's linear frame buffer not by the amount of Video RAM or chipset limitations.
Starting address of LFB:
0xC0000000 -> 3GB available
0xD0000000 -> 3.25GB available
0xE0000000 -> 3.5GB available
You can test this by checking the used memory resources of your VGA in device manager.
If you have an integrated VGA with an LFB address of 0xC0000000 and 256MB frame buffer you will have 2.75GB available memory. If you have 2 active modern VGA the available RAM would be less than with only 1 because the system has to reserve 2 LFB regions. In case of a 6600 this amount of RAM (256MB) can be accidentally the same amount as the Video RAM, but it's just a coincidence.
So even under a 32-bit OS you can use cards with 2/4 GB of Video RAM since addressing video RAM is a concern of the driver. On the CPU side there's usually only a 256MB area in the PCI address space. The same is true for 2 cards.
Regarding video memory handling and the 256MB area: an analogy could be that under pure 16-bit real mode DOS that has an address limit of only 1MB you can use the full VGA frame buffer (8/16/32 MB etc.) through VESA driver/BIOS calls by using a 64KB area (A0000 - AFFFF) and bank switching.

There's nothing bizarre about what you're doing, the only note of caution I'd raise is that 500W sounds rather low for a 780Ti, a 6600, plus everything else.

I wanted to do exactly this in my XP/Windows 8 retro box, but unfortunately it's an XW4600 motherboard that's *very* picky about PSUs, and I'm stuck with a 600W power supply with 30A on 12V, which isn't enough for more than one modern-ish reasonably powered GPU.

Already did this... on a bit larger scale (LINK).
If you can live with 6600 as your secondary GPU, you are fine.
Also

The only thing I'm worried about is Windows XP refusing to boot on a machine with nearly 12GB total memory between System and Video RAM as IIRC video memory counts against the 3.25GB RAM limit on Windows XP.

There is no such thing as counting total VRAM against total RAM pool.
You only get 256MB system RAM reduction per PCI-e GPU (only on 32-bit OS and only if your board doesn't support Memory Remap feature).

Proof #1 :
I present to you, a PC with "negative" RAM (as in, using more VRAM than available system RAM on 32-bit OS) :

FYI : System has 3200MB of RAM available from 4096MB.

Proof #2 :
RAM check on Nvidia GPU (just in case you think they work differently :

Last but not least :
Proof #3 ("Sanity check") :
You wouldn't be able to install GeForce GTX Titan (OG) or Titan Black with 6GB of VRAM, if you had less than 6GB of RAM installed, and NV doesn't mentioned this limitation at all.
Both are natively supported on Windows XP 32-bit : LINK

I hope this will kill the whole 32-bit "VRAM vs. RAM" myth.