Nononono...
400 watts for an 8800 is a death wish. You'll be skating on ridiculously thin ice plugging it in, much less turning it on. I'd recommend something over 530 watts to preserve the integrity of other components.
If you're going to get the 4850s, you may as well wait a little while and get the even better 4870s.
But if you have the cash, go ahead. Just be warned, if you see a 8800GTS 320 or 640, avoid them at all costs. They're based on an older, slower, core.
8800 GTS 640While this may look cool and has had a recent price drop, avoid it at all costs.
MSI 8800GTS 512I'd definitely recommend this, even though it doesn't look as cool or the cooling isn't as efficient, it still runs cooler by itself and pushes a lot more graphics power.
Remember, you have to keep all your parts on the relatively same level. If one component isn't as efficient/powerful as another, you're wasting a lot of time. Even if you have a mid-to-high end quad core processor and 4GB of RAM, but a really bad, I don't know, Radeon 9200SE (which could run Sins, but I haven't have time to check), the RAM and CPU will be waiting 50% of the time for the graphics card to redraw the screen to continue. This goes the same the other way, too. The most common bottlenecks are RAM and graphics card. CPU, not so much.