I recently built myself a new PC with the following specs;

CPU: Intel Core 2 Q6600 quad core processor.

GPU: Sapphire Radeon HD4870

Mobo: Gigabyte X-48

Ram: 4Gb Corsair Dominator 8500

And a nice new 22inch monitor to go with it.

I have to say that so far it has been performing like a beast!
I crank everything up to the highest in UT3 and it runs at a steady 60fps (which the frames are locked at in that game by default anyway!)

I'm still yet to give it a shot in Crysis though, but we'll see.

I'm planning to Crossfire another 4870 in the near future as well.
Thing is, after reading some of these benchmarks of the 4870x2 out-performing dual 4870's in crossfire mode I'm not so sure..

The only thing I do know is that right now ATI are offering a whole slew of fantastic GPU's for incredibly low prices. (4870x2 aside which is their flagship card and costs an arm and a leg)

Also with Intel processors being as good as they are right now and surprisingly cheap!

It's a really good time to be upgrading your PC and with a decent amount of great titles coming out soon, (Starcraft 2, Diablo 3, Space Siege, Dragon Age) there's no reason why you shouldn't!