Looks like a pretty nice system.
The boards 775 socket will handle going quad core down the road. Although for the uses you describe, you may not have to. The core 2 Duo's are no slouches and only some specific apps and games benefit from going quad enough to make it worthwhile. New apps come out all the time though so it's nice to have the option.
Regarding "the new PCIe standard", if you are referring to PCI-Express 3.0, then I wouldn't worry about it at all yet. I don't believe they are actually manufacturing anything that uses it yet.
Buy the fastest RAM you can afford. It's the least known, important factor, that most people don't even think about.
Nice case. Should keep things cool until you get the upgraditis and play with overclocking. Unless you are in a really dusty environment, I'd probably trash the 'washable air filter'. If you are like me and likely to forget to ever check on it, they seem to grime up faster than the inside ever gets dusty. Logic = that only a tiny fraction of dust that blows into your dust actually stays there, the rest blows back out again. However a filter traps 95plus percent of it so buildup is way faster.
My neighbor (same on who got his paypal acount stolen because he won't put in a firewall & antivirus) recently cooked a 5 month old motherboard by ignoring his filter.
