I've got a Seasonic s12 330w, and I'm wondering if it'll power a 8800GS okay.
It's got  2 12V rails @17A, do they combine to power stuff or what?
 

I would say jump up to a 400 watt power supply. I have used a 350 watt to power my 9600GT OC for a couple days with out issues.

If your listed configuration of :  
 
P4 Northwood
2x512 Crucial Ballistix DDR 400
XFX 7600GT AGP
WD 80GB @ Sata 1.5
Seasonic 330W
 
Its possible.  Extremely tight, and wouldnt recommend it.  Single core old P4, i'd work on the rest of your system before the new video card.

no no, I'm doing a whole new mb/ proc/ ram/ gpu
 
Ga-p35-ds3l
e8400
Adata ddr2 800 2x1GB
8800GS

Id say to upgrade the PSU.  Thats too close IMO.  Corsair 450 watt if your not going sli.

the new system specs necessitates a new P/S

Xclio greatpower 500 then.

The more the merrier, within reason that is.  Read the motherboard manual to see what the minimum amps that are required and make sure the new psu meets those requirements...