After looking at the minimum PSU requirements for the HD 5750, I see it's listed at 450w. Would the Corsair 400w be able to handle a Radeon 5700 series card without having to upgrade the PSU.
Not worried about X-firing as I know the PSU is not enough for doing that. Relatively new system and I'd rather not have to get a new PSU just yet. Just looking to get a decent video card without the extra expense.
The 400CX would be enough for that, its a hearty little 400W unit, and much better than some cheap 500W units. With your system you will probably only run it about 70% of max so you have enough additional headroom not to need to worry about it.