Personally, I use the top 2-3 keyword phrases I'm optimizing the page for in my <TITLE> tag... with primary keyword first followed by secondary or terciary. Like the previous poster said think density (as you do in the page content) as well as position (similar to having your most relevant/important info to be indexed near the top of the HTML).
Since title tag carries a lot of weight when ranking your page, I do not include extra words like a call to action. This further increases your density in the <TITLE>.
I save my "Call to Action" for my meta description. In most cases this is what is displayed directly under the page title in the SERPs unless you have an entry for that page in DMOZ.
So assuming "Car Wallpaper" is your primary term and benz wallpapers, toyota cars, sports cars are the 2nd, 3rd, and 4th keyword phrase for the page, I would make the title something like:
Car Wallpapers - Benz wallpaper, toyota cars, sports cars
and then the description which would appear directly under it in the SERPs would have the call to action...
Remember to use singular and plurals if you can work them in like "Car" and "Cars", "Wallpaper" and "Wallpapers".
[edited by: ZydoSEO at 8:22 am (utc) on Dec. 16, 2007]