For now get a decent motherboard and a very good power supply unit. For with memory you'll want four one gigabyte sticks over two 2,048 megabyte sticks for performance. Note, it'll chew up more power doing this. Also note, if you do use four gigabytes, the amount of other sources of memory in your system will detract from this number. This is the reason why there's a lot of threads out there asking why the poster's computer only sees only around three gigabytes of RAM. If you do go up to four gigabytes or more, go with Vista 64-bit.
In regards to DDR3 memory, don't bother for around a year, unless you've got a lot of money to burn. Micron Technology reports that for the first year or so the premium on DDR3 memory will be 80%, while approximately a year after that the premium will be down to 20%.
There was a review recently posted in CPU about DDR2 against DDR3 performance. I can't remember the reviewer's site, but there is an upcoming motherboard with the Intel's P35 chipset that supports both memory types. I believe it's an Intel motherboard, but I can't remember. By August the motherboard and DDR3 RAM should be available. But remember, there is that 80% premium that'll be going around for quite some time.
Hope some of that helps.