The noise isn't that bad, but dat heat. Dat heat.
Personally, I've been considering selling my X58/GTX 480 SLI rig before it reaches the point where it's essentially worthless. It's not that I don't love it. It's plenty enough for anything at 1080p and powerful enough to deliver a good experience at the 5000x900 I play at normally. But after 15 minutes an average size room will be noticeably warmer. At 30 minutes and beyond the room grows progressively more uncomfortable. My case has no sidepanel fans or vents, and the 480's proximity to the PSU dramatically heats the PSU's casing. I have to run with a custom fan profile because if I don't, they hover @90C under load. Sure, that's within Nvidia's specs, but it raises questions about longevity.
If you've got a well ventilated case, game either in an air conditioned room, aren't bothered by heat, or live in Alaska, and have the PSU to handle it, another 480 might be a decent purchase. As others have said, though, I would seriously consider selling your current card and dropping down to a single card setup of a 28nm card. Given the price your liable to get for your 480, it might be somewhat of a downgrade performance wise when compared to 2 480's, but you'll get more VRAM for future-proofing and drop a couple hundred watts. Even at 1080p, I don't suspect that 1.5GB will be enough for high/ultra settings for much longer.