I'm hoping to upgrade my video card to something along the lines of a HD 4850 or 4870 (or comparable nVidia card; I'm not picky about that), but I'm not sure whether or not my PSU can (safely) handle it.
I Think wattage wise you fill be fine, I have a 9800gtx+ (pretty much the same as the 4850) and a 3.2 x2 and I only pull 300 watts on load with a antec eartwatts 280. but you only have 25 amps on your 12v rail and I'm not sure if that's enough, I couldn't find the amperage specs for the 4850.
If the link is correct, what could be the reason for the 4870 consuming less power? Is GDDR5 more efficient than GDDR3 or something?
Also, would it be possible for a card to draw from both rails of my current PSU, or would I max out at 15 on the second rail? I don't think they're actually physically separate rails, but I'm far from an expert on electronics/PSUs.
I don't know why the 4850 is shown as using more amps, there is this weird thing that you can do with the total watts avalable on your 12v rail (found by looking it up or using amps and 12v) and figuring out exactly how many amps your GPU uses so it could have something to do with that, or just more efficeint ( I hear the same thing with the 4890 and 4870 the 4890 being more efficient and more powerful)
anyway aside from that those amps seem low but i don't know ATI cards.
About drawing from both rails I think that is what happens, from my understanding which could be easily wrong is that the rail is just a think inside the PSU that converts the power and lets the amps through, multiple rails were created to avoid a hazard of putting to much amps through and creating a problem, but still allowing High end graphics cards.
One think I know for sure is that my GPU specs for 26amps on the 12v rail and I have two 17 amp 12v rails and it runs fine without any hiccups.
I would recommend an HD4770 instead, it uses about 1/2 the power of 4850 for almost the same performance.
I actually was looking at the HD 4770 as the best option a few weeks ago (largely due to power consumption, as you point out), but the availability seems really low now, and with the dropping prices of the 4850 and 4870, I've actually seen it cost more than the 512 versions of those cards. For several days, Newegg didn't have any 4770's available at all, though I think Tigerdirect did have 1 or 2 models.
Looking at the rest of your build you should be o.k with that 400W PSU.
With either model card that was mentioned? If I can safely run an HD 4870, that's what I was planning to go for (due to current prices), but if I "need" to go with a 4850 or 4770, then those are certainly options as well, and I'm sure all of them would drastically out perform my current 2600 XT .
Thanks for the replies so far; I'm hoping to get something ordered in the next week or so, but I want to be sure I can actually run the hardware safely without upgrading my PSU for a while yet.