Newbie has questions regarding the 'shelf-life' of seeds!

Hi All...
I'm fairly new to gardening... In the past years I have planted annuals in containers and a few things straight into my 'already developed' gardens. This will be my first year putting my all into gardening. I've decided that I'd like to start with seeds. I have a couple of packages of seeds that are unopen and older (per their date on the package) and open and older. My question is: What is the shelf-life for a package of seeds? I hate to throw away something that is perfectly ok to use. For example... I have four packages of seeds that are in question...

Tomato (Big Boy Hybrid), Open - sell by date 11/2006
Daisy (Shasta Alaska), Unopen - sell by date 11/2006
African Daisy, Open - sell by date 12/2007
Marigold (Queen Sophie), Open - sell by date 11/2007

Also, I want to plant the tomatoes in an upside down container... I was told that the 'Best Boy Hybrid' were the best... Is this true? I'm having a hard time finding that specific hybrid. Would Big or Better Boy work just as well?

Any feedback would be greatly appreciated!! Thanks :)

If seeds have been stored somewhere cool and dry many of them will last for several years, and since you already have the seeds there's really not much to lose by trying. If they've been exposed to moisture or temperature extremes though the chances they're still viable are not as good. Typically your germination rate will be a bit lower the older the seeds are so you might want to sow them a little more thickly than you would have otherwise.

Thanks, ecrane3! I appreciate the feedback...

Anyone know anything about the tomatoes??

I like to start my seeds indoors very early. Most stores don't even have seeds in yet, so I can't find what I want when I'm ready to plant. This year I bought seeds for the plants that I plan on doing next year. Some years I have lot extra (when I end up seeing them on clearance for real cheap) I've never had a problem with older seeds, I just sow a little thicker. Seeds are a part of nature, expiration dates are made by companies who want to make more money!
Tomatos- I start my seeds inside and the plants do better than my neighbor's store bought plants. We have a chain link fence between us and they are planted right next to each other. They get the same amount of water 'cause I can't set my sprinkler to not spray his plants. Hmm- I bet his water bill is cheaper than mine lol! I've never tried the upside down container, but I did try putting a tomato plant in a very large pot. It did not do as well as the ones planted in the ground. It hung over the side, but it never looked very happy.
Marigolds seeds usually take a long time to flower, so start as early as possible. I've started marigold seeds many years and usually they germinate very well. I've had old store-bought ones, ones I saved from the yard, etc. I've had a little problem with them germinating this year, but with patience finally have all of my 'spaces' filled. (My seed trays) If they give you a problem this year, just keep trying.
I did Shasta Daisys one year but they didn't transplant well into the ground. All of them died. These you might want to direct sow. This is the first year I've tried African Daisy. So far so good.