If a motherboard says it's memory standard is DDR2-800 will it only support 800mhz memory or will it support anything less than that, say, the DDR2-400 memory I have?
Yes, if they are both DDR2. But just watch out, since there is DDR 400, as well as DDR2 400.
Bleh.. trying to replace a motherboard without changing any other hardware is nearly impossible.
Well.. when you have a Pentium 4...
Yep, I believe DDR2 is 240 pin. Faster = better, and more memory bandwidth (the speed) is more important than tight (low) timings. I have 2 GB of DDR2 800 at 4-4-4-12 timings, and I usually have quite an excess. All I can say is that my memory has never been a bottleneck - something I could not possibly say when having only 1 GB. It's so awesome because over the years I've gone from 64 Mb to 512 Mb, 1 Gb, and now 2 Gb of memory. I honestly doubt that I would see a performance increase if I doubled my memory with what applications I use.
Honestly, if you're running anything less than a gig of that memory, I'd ditch it and buy at least 1 GB of DDR2 800 - the current sweet spot of DDR2 modules. Think of DDR2 800 as the DDR 400 of last generation. After DDR2 800, the price/speed ratio grows exponentially.