Here goes assuming that you have a motherboard that supports SLi (!)
First check the spec of your current power supply - there's a sticker on it and assume that it's optimistic!
Expect to draw 40-140 amps power just for the cards so you will need a powerful high efficiency PSU which will cost you over £100+ but get advice from a specialist gaming site
A standard PC can consume 400w depending on your cpu/ram spec plus other hardware and accessories.
Plus if you work on video/audio or want to play high resolution games you might add the same power requirement again as a safety margin for the so it's going to be an investment on top of the cards and a compatible motherboard.
Some advice on power for each card here:
Read the spec/requirements for all the items you mention - if you are building your own make sure you have more than the recommended minimum, plenty of cooling apparatus and that the two cards will fit in the case!!
8800 gt spec from Nvidia site
Standard Graphics Card Dimensions:
Height Single Slot
Length 9 inches
Width 4.376 inches
Thermal and Power Specs:
Maximum GPU Tempurature (in C) 105C
Maximum Graphics Card Power (W) 105W
Minimum System Power Requirement (W) 400w
Supplementary Power Connectors One 6-pin