Its very outdated. Extremely hot and power hungry for the performance. Not to mention the usual dual gpu problems. You'd be better off with almost anything else from about an HD 6870 on up. May not be necessarily faster, but modern single gpu cards are a lot more efficient.
While the Radeon HD 4xxx series runs a bit hot it should be fine as long as you have enough airflow.
The HD 4870 x2 512MB is a little dated, but it is not outdated. It does not need to be immediately replaced if do not have the $$$ to buy a card to replace it with. You will still be able to play many current games with high graphics quality, but the 512MB of VRAM does limit performance a bit if your monitor is 1920x1080 / 1920x1200.
The 4870 X2 is still a solid performing card. Since you have been running it for some time now, if there were any issues, you would know about it. Take the time to blow the dust out of the card while you have out, and it will probably run well for quite some time to come. That new motherboard, memory and processor is going bring your 4870X2 to a level of performance you would have never believed was possible. Run it for a while before you dump money needlessly on a newer card, I think you are in for a pleasant surprise.
I am currently using the Sapphire HD 4870x2, Gigabyte EX58 EXTREME, i7 950.....works great.. was thinking of maybe dual 5770 or 6850 but the card is stable even with the Overdrive ramped up. I can wait awhile longer for upgrades.
I would just keep what you have. An upgrade from what you have would cost some dough, and what you have is nice. All of these people saying that it runs hot and its old are right, but so did the G92 core Nvidia cards, like the 8800GT, 9800GT, and GTS250, which I still have. As long as you have good airflow and take care of it, it will take care of you. When your card starts to reach the bottom of that chart, then you might have to worry, but that wont be for awhile.
I would go with the 2500K, it would be a great addition to the 4870X2. You will be surprised at what it can handle I assure you.
If you can make sure the airflow is great in your computer. If you find card overheating, then you can either crank up the fan inside the GPU or take other action, like my friend has 2 8800GT's in SLI, and he has his desk fan blowing onto them with the side open, and it works.