For a system like that, any decent PSU should be ok, I think that even a small format 90 W should be able to provide the power, if it's not too aged (a PSU tends to loose power in time). A 200-300 W should be more than enough, but try to stay away for generic brands. Brands like Fortron, Chieftec, Enermax, etc are always a good choice.

I don't know the exact power requirements for a Voodoo2 card, but I doubt it's more 20 W, the PCI slots can't provide too much power, and since V2 doesn't have any other way for taking power, it's limited by the PCI slot. PCI 2.1 is 3.3 V, some implementations allow 5V, but I'm 99% sure that Voodoo2 use only 3.3V line.

Unless you plan to put in a very small PSU, I'm confident your Hard drives will consume more power than the SLI setup.

Back in these days, the cards didn't draw significant amounts of power (couldn't do anyway, as they were limited by the PCI bus power supply), therefor I don't see any problems with any halfway decent PSU.

PCI Bus electrical specifications :
Code:

Supply  Min. Voltage  Max. Voltage  Max. Current  Max. Power 
+3.3V	    3.00	  3.60		 3A	     10.8W 
+5V	    4.75	  5.25		 2A	     10.5W 
+12V	   11.40	 12.60		 1A	     12.6W 

The Voodoo2 doesn't even come near using all the power as far as I know, never heard of power related problems in these days.

The Peripheral Component Interconnect Special Interest Group (PCI-SIG) is a good address :

http://www.pcisig.com/specifications/conventional/

Just buy one good 365 or 400w powersupply, then when in future you would like to try out some other platforms, you can just use that one. Bit of future proofing never hurts.