The 4770 was never really designed for 1080p gaming, it just doesn't have the bandwidth or memory for it. If you run out of memory or bandwidth your performance can drop off a cliff quite sharpish and if you plan to play at 1080p that situation is bound to turn p moe often than you'd like. Having just made the switch from a 4850 to a OCed GTX 260 (216) myself I can say the gap is pretty damn huge at higher resolutions. Don't get me wrong its going to be a great card and for gamers playing at 1680x1050 and below there's not too much need to spend anything more really, but if you've got a high resolution panel, you should at least think twice about it, anyway.
Far Cry 2 has got to be worth at least $10 to you so for an extra $50 outlay you're getting a significant leap in performance without the worries of running out of memory or bandwidth and you can pick up the card a whole month earlier. Well worth it imo, but the choice is yours. Oh, and sorry if I'm tempting you to spend money you really don't want to, I've fell into the trap plenty of times myself!! :lol :lol Never regretted it once though.
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I'm with Victim on the PSU front as well, considering how cheap your getting that BFG PSU for its worth it for peace of mind alone. The 4870s have a 2 slot cooler, so you'll have to leave the slot immediately below your PCIe slot vacant. So long as that's not your only PCI slot, you should be OK, but again check its not too long either, you'll be stunned at just how big some of these modern GPUs are in the flesh.
With regards to idle temps, even if they were a good deal higher, I wouldn't worry about it, its load temps that actually matter.