If you are gaming then no if looking to render/encode then ya moving to i7 makes sense As for SSD - personally i am gonna move to a DDR3/SATA3 platform later this year so am eyeing the SATA3 SSDs but to datte only Corsair has one and they pricey hehe Other wise look to hit a 64GB Kingston V+ series SSD?
But really the only thing on there that would be worth it would be an SSD.
See if there's something wrong with your graphics card, possibly RMA it, or just find a way to make it cooler so it doesn't have to fan as much.
That stuff is still fast enough that there's really no point to CPU/GPU upgrades. You'd have to wait a year or two to make it worth it. Unless of course you hate the 4870 x2 so much that you'd ship it to me (I'd pay the shipping) and want to buy a card anyway. I'd go with the 5850.
I wouldn't do anything with rig you have. If you hate the GPU you have, and money is burning a hole in your pocket, maybe get the 5870, maybe, but it won't be any faster really, just quieter. For gaming purposes, spending all that money on an i7 will net you very, very little in return. Not worth it at all. If you do a lot of video encoding, then yes, the i7 will make a difference, but I would have to be doing a lot of it to justify spending the cash even for the gains you get doing that.
If you want a quieter GPU then you may want to consider putting an aftermarket cooler on your card. A Radeon 5870 will provide you with about an equal amount of performance, so that's alot to spend just to have your GPU a little quieter. Instead you may want to consider slapping this on your 4870x2