I would think that 500 watts is a minimum these days.
Well that unfortunately is a wide spread rumor which is not true. When using just one graphics card you'll rarely need more than 400-450W (that excludes those two chips on one PCB cards because they of course have a higher power consumption too just as the same disadvantages of such dual combos). The only thing you have to take care of is to actually use a quality power supply and not like let's say one from LC Power who really produce quite a lot of poor supplies.
For stability reasons I would still recommend a power supply of about 500-750Watt. A HD4870 and Q6600 is a different story than with a Phenom 9850 (a processor I wouldn't recommend as it's not quite as fast as Intel's competitor).
As far as mainboard goes, you'll have to think about whether you want to jump on the DDR3 bandwagon any time soon. The newer RAM is worth it in terms of performance, however it's mad expensive to get even just 4 Gb in comparison to DDR2 ram.
I personally wouldn't bother buying a Vista 64-bit OS, but word has it it works better and better in terms of programs supporting 64bit.
The statement that games still struggle with dual cores and quad cores is total nonsense by the way. All Xbox360, PS3 games and just about every PC game released after 2006 are able to handle dual cores and quad cores just fine, taking full advantage of them. They really use multi cores more and more effectively, some games do this better than others, but you'll notice heaps of difference between a Quad Core running Crysis and Dual Core running Crysis... In fact there's usually a massive 40% to 60% increase of performance. Price-wise you can't go wrong with a Q6600 or newer quad core either...
Cheers
