@CPU:
If a Quad Core makes sense for you depends on what software you actually use. You'll only find a broad support for this on sectors like Rendering, Encoding and things like that. Games and most other pieces of software still struggle to use two cores for good and that will also change in a rather slow manner (although when talking about games it may go faster on the RTS and racing genre - don't know if you like to play those games a lot)...

On the other side when talking about games the GPU will become the bottleneck first anyway so you shouldn't worry about the CPU too much. So until you've made clear in which way you want to use your PC there hardly can be made a recommendation on this...

@Mainboard:
Although a bit more expensive I'd go for an Asus P5Q Pro featuring a more modern chipset (slightly upgraded features and less power consumption) plus a way nicer board layout...

@Power Supply:
Any quality power supply above 400W should be enough here...

Final thoughts:
Well there are two things I want to add here:
1.)
If it's possible I'd wait a bit longer because Intel will release their new "Nehalem" CPU generation which might be quite nice but features an entire new socket & chipset so you won't be able to use any of those CPUs if you buy a system now.

2.)
I'd really start to think about using an AMD system. It has quite some advantages: You can get a good mainboard and memory a bit more cheap and their CPUs aren't that bad and give you a good price performance ratio. The main advantage is that you'll be able to use new CPUs with that system for quite some time as they will be downwards compatible. That might be extremely interesting when looking at how long you've kept your current system - you could do a decent upgrade that way without buying an entirely new system...

Enjoy your meal
Toast