Will my PSU run the 7900GS graphics card?

you can calculate the power you need here :  http://web.aanet.com.au/SnooP/psucalc.php      

Okay:
First the 22 amps given on the video card documentation isn't additive to what the system already is, it's an assumption of the total needed for the system and the card. (That's the good news, now the bad.) Looking at a few 6800XT the number you got is pretty low, several others say 28 amps. I'm going to use the 28 amps.

Next (I'll add a pic below) despite what PSU advertisers want you to think there is essentially no such thing as separate 12v rails. They are separate in the regulation stages but jumpered together at the output stage of the PSU. (It is being called 'virtual rails' by some.) The only model of PSU by anyone that has legitimate split rails on the output only comes in over 1000 watt sizes and is in the $300-$350 price range.
The smallest wire I've seen used for those jumpers is good for 7-8 amps but I'll say 4 amps to be conservative. (Most of them have more than one jumper but I'm sticking with 4 amps anyway.)
- SO: 12v1 and 12v2 can send each other up to 4 amps to compensate for what looks like insufficient amps on the label data for those rails.

Can't just go by total watts anymore.
There are too many different board and PSU configurations and you need to look at the amps on each voltage these days.
The +5vsb for instance is a critical rail. That's the one that's 'on' 24/7 and is responsible for starting up the machine. Not one that you want overloaded and it might be overloaded 24/7.  

An Intel D915PBL has the same chipset and features (sound chip / LAN) as your board.
Intel's Technical Product Specs for their boards give a fairly detailed break down of the power consumption of a motherboard:
It's here:
http://download.intel.com/support/motherboards/desktop/d915pbl/sb/c6859901.pdf
Look under: Technical Reference -> Electrical Considerations -> DC Loading
In table 34 the 'Maximum Loading" numbers assume the CPU, RAM, and USB are loaded to the max the board is capable of but do not include the power used by cards in the add-in slots.
That's a good place to start because you know you will never go over that (without damaging the board).
That gives
+3.3v - 6 amps [IC chips, add-in card IC chips]
+5v - 14 amps [USB, Drives, IC chips, add-in card IC chips, RAM on some boards]
+12v - 16 amps [CPU, Drives, IC chips, add-in card IC chips, RAM on some boards]
-12v - 0.1 amps [IC chips, add-in card IC chips]
+5vsb - 1.4 amps [Standby Power, RAM on some boards]

Assuming the DVD is not a DVD-RW your HDD and DVD add about 2 amps to each the +5v and +12v

+3.3v -> 6 amps
+5v -> 14 + 2 = 16 amps
+12v -> 16 + 2 = 18 amps [over ridden by 28 amp assumption for video card.]
-12v -> 0.1 amps
+5vsb -> 1.4 amps

+3.3v x 6a = 19.8 watt
+5v x 16a = 80 watt
+12v x 28a = 336 watt [CPU rail needs 14a to 16a of this]
-12v x 0.1a = 1.2 watt
+5vsb x 1.4a = 7 watt

-> 444 watt

That's for your system in max (peak) power condition.
Most of the time you will be using less than that.
I would be looking at:
450 watt (or over) PSU's labeled for peak power.
400 watt (or over) PSU's labeled at nominal power.
[Usually only high end brands list nominal power.]

Make sure you check that the ALL OF ...
combined +3.3v/+5v
combined +12v1/+12v2
and +5vsb
... have enough amps.
And that the two 12v 'virtual' rails are within 4 amps of what is expected AND that the other 12v rail has enough room left to provide it.

.