The RAM in question is OCZ Platinum XTC PC2-6400 4GB 2X2GB DDR2-800 CL5-4-4-15 DDR2 240PIN Dual Channel Memory Kit.

That would be 4 GB. I want a lot (actually I'd prefer 8 GB, but everybody warns me off that much). This is for very very heavy multitasking, but not gaming. I'll be adding a 22" LCD monitor. I want to be able to look at files holding tens of thousands of .jpgs, going back and forth without hourglass waits. Also I scan the huge astronomy pictures. As well, WORD documents of hundreds of pages, fully formatted, with many .jpgs imbedded in gigantic tables. Meanwhile, virus scans and music. All that at once. Just no gaming.

OS: XP Pro x64. (Possibly Vista x64 later, but I don't like Vista.)

CPU: AMD 64 x2 5200+ Windsor F3 EE, 65 watts, nm90.

mobo: probably, almost for sure, Gigabyte GA-MA78GM-S2H with the new 780G chipset

That chipset includes onboard video, which can actually be crossfired with a video card. That would be:
Radeon HD3200-based onboard graphics
+ a dedicated ATi HD3450

Opinions are welcome. Meantime, I'm continuing searching and researching. Note: I have been advised against this chipset, which I was told would not be fast enough.

Thank you.

No pictures....how can I tell you if it looks good!

OCZ is just what the name states. It is overclocked memory.
When you choose a motherboard make sure you have the option to adjust memory voltage along with memory timings.

OCZ Platinum XTC PC2-6400 probably has the SPD programmed at DDR2 667 at 1.8V with timings set at 5, 5, 5, 18.

4GB RAM looks good to me. Anyhow, my suggestions for RAM:

1) should you know the memory usage for your applications (e.g the words doc + viewing jpeg etc), then the total minimum memory that you will need would be = total amount used (average) x 1.5. Let's say that normally you will need 2GB mem, then multiply with 1.5, you got 3GB as your minimum req.

2) regarding this OCZ, I read that it has some issue on some mobo. Try to see Gigabyte sites for memory compatibility list. And since it's AMD processor, which is the memory are tied to the Processor, you should do some research on proc+memory compatibility as well.

I haven't use AMD yet, but that what I was read or told. IMO 4GB should be fine, but if you find that you need more, you can always add it later. And you should go for any 2x2GB, so that you still have 2 slot left for additional mem later on.

For the mobo itself, it's a new mobo, so there are still few reviews on it. I didn't see any negative issue yet, but I don't know. To me it's a good board especially for anyone who want to use it for HTPC or light gaming. Which is more or less will be also suitable for you. So 780 will do IMO.

If your budget allows go find a faster processor. Your choice is OK, but I think you can get a faster one. I'm saying this since you're mentioning that you'll need this PC for heavy multitasking, which means it will be relying on processor for its task.

Cheers

Thanks to both of you. I had actually not realized that RAM was overclocked, despite my looking straight at the OC in its name.

I have selected that processor because I want AMD, and it has the combination of 2 x 1024 L2 caches and 65 watt operation. Below that, the series comprises 2 x 512 L2s. Above that, the series is 90 watts.

I'm aware the Phenoms are trying for better with L3 caches, so I've intended a mobo that would support a Phenom in future.

Now I have another question: if I try for 1066 RAM on what is really a DDR2-800 system, I've been told by an overclocker that it is a very simple matter of OCing the processor. I would want this, if that were the case, for day-in, day-out operation. Is it a "very simple matter"?

I used to work as a naturalist-guide in the Rockies. Bighorn sheep were plentiful there. I thought I knew all about RAMs then. How times change.