What kind of DIMMs are you using to attempt 4GB? Both how many chips of which size is relevant, as is the exact vendor/model.

As for instability, the nForce2 is notoriously picky about RAM compatibility, even with correctly specced DIMMs. That said, I alway had the impression that Gigabyte's designs were a lot more forgiving than others. I had a 7N400L back in the day and my housemate an Asus A7N8X-E. Basically the same board, but mine would happily take any combination of the PC3200 DDR-400 DIMMs we had, whereas his only managed to boot with one specific pair (mine of course - but as everything worked on mine I was happy to swap).

I wouldn't assume whatever issues you are encountering are Win7-specific, particularly not if they seem temperature-related. That's hardware playing up one way or another. You'd probably have the same problems with any other OS. If you have sufficient RAM (and 2GB is sufficient), Windows 7 actually has a smaller CPU footprint than Windows XP, particularly if you strip unnecessary bloat off, so if you want to do something practical with the beast (as opposed to running it for period applications), I'd stick with Windows 7 and focus on sorting out whatever is causing the instability.

With any boards this age: check the capacitors, particularly the ones around the VRMs.

Try memtest86. I've seen some of what dionb mentioned - I have a large number of identical DDR400 modules which work great in Intel 865/875 boards, but which always give errors on the nForce2.
I think that when the DDR400 capable version of the nForce2 was introduced, there wasn't yet an established JEDEC standard and so things got a little dicey with that type of RAM. I've had the impression that JEDEC didn't like the idea of DDR400 and was late to give in and standardize it.

Gigabyte boards of that period tend to have marginally cheap capacitors on them like OSTs, which are likely to have gone bad.
If you end up replacing caps, notice the empty pads around the CPU. If you measure continuity you may find that they're on the same circuit with some of the caps that were populated, so that gives you more options on what to replace the originals with.
Many of their boards at the time were laid out for a larger number of Vcore caps than what got installed.

Wireless wrote:

Mmm...bought two of these, they are supposed to support Socket 462 and 4GB DDR400 (using double data rate at 200 MHz FSB), but i […]
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Mmm...bought two of these, they are supposed to support Socket 462 and 4GB DDR400 (using double data rate at 200 MHz FSB), but in reality seem only to recognise 2GB even with the latest version FK Bios, I bought them to run Windows 7 Pro 32 bit, and an Athlon XP 3200+, for a practical machine recovering data from U2W SCSI HDDs via an Adaptec 2940, but it seems this is not the system board for that application.

Despite the 3200 being able to run up to 85C Max, the board seems unstable above 60C CPU and 30C System Temperature when running Win 7 Pro.

I may instead run Windows XP SP3 Pro, since its supposed to be optimised for it, but I disappointed that Windows 7 support is so poor for such a late board.

I'll be keeping the one board, putting it in a cheaper case (currently its in an Antec P100 from circa 2005), to run a very quick XP machine, but will be selling the second board, if you're interested in it, let me know.

nForce2 motherboards will support a maximum of 4GB of ram per 4 slots. If it only sees 2GB when 4 sticks are populating all 4 slots then you are using high density ram.