Actually what matters most is to have a balanced system. For example, having a 5970 won't do you much good if you run demanding games on a dual core with 2GB of RAM, as the PC is like a pipeline that moves data around from one component to the next: it's only as fast as it's slowest component, because if the components don't match well in speed you have bottlenecks where one of them is waiting for data from another, instead of moving data back and forth at a more stable flow.
Sure, this all depends on what kind of games you run as well (what their dependence is on CPU or GPU, etc), but overall it's better to have a balanced PC instead of one with 1-2 components that stick out way above the rest (unless you are in the process of partially upgrading it that is).
This gives you the most amount of usage for all this processing power you buy, plus it makes your upgrade planning and funding easier, because you scale down the components to match and you end up with some leftover cash. Then, 6-8 months down the line when prices drop on some high end components due to the release of a new line, you can use that cash to do a mild upgrade.
I'm running a stock i7 920 at 2.7GHz (i got it so early that it's a C0 stepping batch of CPUs, instead of the easier to overclock D0 batch), two 1.5TB hard disks, 3GB of RAM and an Ati 4890 1GB on a P6T Deluxe v2 motherboard and a 700W Heroichi Cougar PSU.
I didn't buy more RAM because i couldn't use it at the time (W7 wasn't out yet and Vista was terrible at that point in time, so i stuck with XP), i didn't buy a fancier graphcs card or SLI/crossfire it because i was stuck with lower RAM and so on.
However, this system runs all i need to run today, plus it is a good, solid base to build on by changing only a few components. What i'm initially going to do is get another 3GB of RAM for SoW. Then, i might pick up a 5870 around spring-summer when prices are low enough.
In fact, i'm not touching a DX11 card until i can get what is today's top line of cards (Ati 5870/nVidia 480 type of cards) for 200 Euros or less. Finally, when sandy bridge CPUs are out and i7 prices start to drop, i will wait until i can get a six-core i7 really cheap and plug that in the mobo. By that point in time i'll have a PC that is one step below top components for the time, but bought for much less, which will enable me to save money towards my next round of partial upgrades after a further year or two.