Smithwicks can be dispensed with pure CO2 or with Mixed gas (N2+CO2), but the mixed gas is different than what is required for stouts like Guinness. I use a 60% CO2 and 40% N2 mix when I dispense Smithwicks at my house. The mixed gas prevents over carbonation when you get near the bottom of the keg. McDantim industries has a great spreadsheet (pasted below) to help you calculate your setup based on the CO2 content levels at which different beers need to be kept.
For example, I dispense Smithwicks at 22-23 psi with the 60/40 mix mentioned above assuming 38F liquid temp and 2.3 vols of CO2 per volume of beer as is Bass Ale.
When I dispense Guinness, I use 32-35 psi with 75%N2 and 25%CO2 mix assuming 38F liquid temperature and 1.2 vols of CO2 per volume of beer which is roughly the desired CO2 content for most stout beers.
The benefits of mixed gas are usually only noticed near the end of the keg. With mixed gas, you'll actually be able to drink the last 10 pints in a liquid form instead of having to pour them into your tub for a bubble bath.
Cheers,
Jeff
http://www.mcdantim.com/calculate.htm