Well, a lot of that would depend on what you will be wanting to keep in the tank. You may want to do a little research, maybe look around liveaquaria.com for some things that interest you. Do you want a reef, or just a tank with fish and live rock? Give us a few more details about what you want and we can probably help you more.
I have a 29 gallon reef right now. 130 watt PC lighting (soon upgraded to MH) with LPS and soft corals, 2 clownfish. Of course though, the bigger the better. A 29 gallon is not that big for a sw tank. So if you want a tank with a larger emphasis on fish, I'd go with atleast 55 gallons or larger if you can.
Hope this helps.