XPS8100 with ATI 6870 - PSU Requirement?

From what I gathered from a quick search the XPS 8100 comes with a 350W PSU. Currently you probably need 300W to run what you have. To run what you want, probably a 400W. Going higher for some breathing room is recommended, necessary if you want to do any sort of overclock.

The HD 6870 is going to draw at least 13.5 Amps from the +12 Volt rails under maximum GPU load.

The 350 Watt power supply unit I'm guessing will supply a maximum of 25 Amps from the combined +12 Volt rails.

That leaves you with 11.5 Amps on the +12 Volt rails to power your CPU, motherboard's chipset, hard disk drives, CD/DVD drive, sound card, capacitor aging which isn't enough.

You need to add to your upgrade budget for a name brand (e.g. Antec, Corsair, Enermax, PC Power & Cooling, Seasonic, XFX) power supply unit. I would suggest at least 450 Watts or more.

So should I even add those components in? Is the performance increase that significant if I add the hard drives and the video card over the stock stuff mentioned above?

I just opened the case and it looks a bit more complicated than I though to add that stuff in. There is no room for the two drives and the GPU/PSU are not that easy to replace.