Thanks everyone for input. I realized that my APC UPS shows the power usage status. When idle, it seems the PC is using only about 100 to 130 W. Need to test it under heavy load but it seems it should be well below 305W of PSU. Or is there a big headroom I should reserve for because power rating of PSU is highly inflated a la contrast ratios of LCD display or wattage ratings of audio power amplifier? To go with two identical card means I have to ditch the current ATI X600 card which sits on the lone PCIE-16x slot and buy two new cards for slow PCI slots I have. I am not sure I want to go that route at this point yet. I will keep that in mind though.
For trading setups, computers usually run at very near idle speeds. With your 130W current idle speed + another 20-30W for another video card, 305W PSU should be enough. To run 4 monitors, you'll have to run 2 of them from PCI anyway... which is actually OK.. you won't see any speed difference vs. your PCIEx16 monitors. ATI isn't as friendly about "multi-card" as Nvidia... perhaps someone who's got 4-monitors running with an ATI in a PCI slot will chime in. If you have to go the Nvidia route, a Quadro NVS 290 or 285 PCIEx16 + NVS 280 PCI should work.
Only the video cards put demand on your power supply. The monitors have their own plug and do not put a load on the power supply.
Folks, I bought a GeForece 6200 PCI card ($30 after rebate) and it's working fine in tandem with the ATI Radeon X600 card on PCIE-x16 slot. Not much increase in power usage. Still under 130W under normal trading use. At first, I tried a GeForce FX 5200 PCI but I found out it couldn't handle 1080p resolution via DVI, only analog. But the killer was the fan noise. It was much higher than the PC itself. Majority of newer cards have fans and if they all make that kind noise, I don't know how people tolerate it. 6200 is passive cooling so no noise at all. I am a happy camper now.
Both Matrox and Nvidia have lines of "workstation" cards with passive cooling... some support up to 2560x1600, DVI.