Big Names on the Market
I'm in the eve of such a startup, sometimes being small is a definitive advantage. If you believe you can use that not only price-wise but being agile, doing the core job maybe even better than the big names. If you up for that, I think you can easily infiltrate the market.
Job Dependency
Depends of what you do? If you are opening a next-digg or YACW2A (yet another cool web 2.0 application) then stay in your day job, because generally you can do both, especially if you got your friends with you. If it's a bigger scope you might want to stick with your day job until you got a almost there product.
Don't forget, also you can find an investor, sometimes it's best way to go. So you can just quit and still have a salary in your own job.
Verbal Confirmation
It's great that you already got couple of potential clients, now you need to look into and make a business plan. Understand your monthly cost including salaries, and see what percentage of it you can get out of these clients. If it's good then get some more certain answers from these clients and go ahead. If possible establish the company beforehand and get them buy the product. (one of your friends can do it, not all of you need to leave your day job straight away)
Product Development
Being the big market means do the core functionality perfect, it should just work, and it should be easier. Price by itself can not justify a buy unless you get the core functionality right. Ignore useless enterprise features, or any useless feature. You need to be aware that you got so much more limited resources than your competitors therefore 20/80 Rule (Pareto Principle) is for you. Do not try to satisfy 20% of the market by including crazy features, stick with 80%'s requirements. Big players can satisfy or can try to satisfy 100% of the market, if you try to do the same thing you gonna fail miserably.
Finally
Read Getting Real, Do not follow religiously but this book will give you good ideas and will explain advantages of being small.
You didn't mention, but I wanted to write. Make a proper agreement between you and your friends, ensure everything is in the paper before doing anything! I've seen so many similar startups fu*ked up before even start because of this.