^^^ best advice in this thread.
Also, if you're running PPC go in with enough of a budget to bid high from the start. If you try to start by lowballing your bid prices it's going to drag down your click through rate (CTR) because of high impressions but low clicks, which then means that your actual cost per click (CPC) will remain a lot higher than it could be for each and every click you get from that point forward. I learned the importance of this the hard way.
I've found with Google, Yahoo and MSN PPC you want to bid to get the #2-4th spot in the listing for most niches (but not all). Don't bid for the #1 spot because it's a lot costlier and generates a lot of bounced clicks without the high conversion rate.
Like Saint said, read some free reports about PPC and PPV so that you don't lose your shirt right from the start.