The only tri-channel ones out there just now though are the ones that go with the I7-900 series, and beyond the entry level 920 once you get into that kind of range, the price/performance ratio gets blown out the window. You can achieve some phenominal benches with a system like that, but unless you have a specific reason for needing that kind of power you're not likely to see much benefit from it.
As much as it pains me to say as something of an AMD processor fanboy, the best value for money right now seems to be the core i5s. The Phenom II X4s are always good too, and there's supposedly a new lower-power version of them due out soon as well as some decored X6 models.
DDR3 is the way to go generally now that the prices seem to have swung, but so long as you've got a decent amount of RAM a good AM2+ mobo will definately do you well for a long time yet if you're upgrading.
As for the graphics cards, DX11 is still new enough that its all a bit muddled. The ATI cards have a dedicated tesselator that's identical across the entire range, so on certain applications that might rely more heavily on that in the future it could well bottleneck them. If your motherboards capable of it, the 5770 and potentially adding more in Crossfire down the line seems to be the best option just now - in some of the DX11 benchmarks they literally scale by 100% since its the tesselator that's the bottleneck.
As for nvidia, well... there's only their high end stuff out just now and its not exactly refined. There's a secondary chip (GF104) due relatively soon that a lot of people have hopes is going to make the mid range (read: sensibly priced) market a bit more competitive again. I personally have an SLI board (780a) and my plan is to get one of the decent mid-range cards with an SLI upgrade path for futureproofing, but its kind of dependant on them not requiring a nuclear reactor and its coolant attached to run the damn things.
